Surface structures for enhancement of quantum yield in broad spectrum emission nanocrystals

ABSTRACT

Disclosed are inorganic nanoparticles comprising a body comprising cadmium and/or zinc crystallized with selenium, sulfur, and/or tellurium; a multiplicity of phosphonic acid ligands comprising at least about 20% of the total surface ligand coverage; wherein the nanocrystal is capable of absorbing energy from a first electromagnetic region and capable of emitting light in a second electromagnetic region, wherein the maximum absorbance wavelength of the first electromagnetic region is different from the maximum emission wavelength of the second electromagnetic region, thereby providing a Stokes shift of at least about 20 nm, wherein the second electromagnetic region comprises an at least about 100 nm wide band of wavelengths, and wherein the nanoparticle exhibits has a quantum yield of at least about 10%. This abstract is intended as a scanning tool for purposes of searching in the particular art and is not intended to be limiting of the present invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims benefit of, U.S.application Ser. No. 12/750,039, filed Mar. 30, 2010, now abandoned andclaims the benefit of U.S. Application Ser. No. 61/165,283, filed Mar.31, 2009; which are hereby incorporated herein by reference in entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under GrantDEFG0202ER45957 awarded by the United States Department of Energy andGrant IIP0646322 awarded as a SPIR Phase 2 grant by the National ScienceFoundation. The United States government has certain rights in theinvention.

BACKGROUND

In response to ever increasing energy demands and subsequent costs, atremendous emphasis is being placed on energy-saving solid-statelighting devices in the form of light emitting diodes (LEDs).Specifically, a need exists for pure white-light LEDs as a moreefficient replacement for conventional lighting sources. Switching tosolid state lighting would reduce global electricity use by 50% andreduce power consumption by 760 GW in the United States alone over a 20year period. Solid-State Lighting http://lighting.sandia.gov/; ThePromise of Solid State Lighting for General Illumination,Optoelectronics Industry Development Association, Washington, D.C.(2001).

The complications associated with design and fabrication of such deviceshave generated great interest in developing white-light phosphors thatdo not depend on complex doping schemes or combinations of materials.One proposed solution is to use a mixture of semiconductor nanocrystalsas the intrinsic emitting layer for an LED device. Achermann, M.;Petruska, M. A.; Kos, S.; Smith, D. L.; Koleske, D. D.; Klimov, V. I.Nature 2004, 429, 642-646; Mueller, A. H.; Petruska, M. A.; Achermann,M.; Werder, D. J.; Akhadov, E. A.; Koleske, D. D.; Hoffbauer, M. A.;Klimov, V. I. Nano Lett. 2005, 5, 1039-1044. Semiconductor nanocrystalscan exhibit high fluorescence quantum efficiencies and large molarabsorptivities. Alivisatos, A. P. Science 1996, 271, 933-937; Yu, W. W.;Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 2854-2860. However,mixtures of such nanocrystals merely approximate white light by mixingthe traditional red, green, and blue colors and result in a loss intotal device efficiency due to self absorption for a device of more thana few monolayers. Mueller, A. H.; Petruska, M. A.; Achermann, M.;Werder, D. J.; Akhadov, E. A.; Koleske, D. D.; Hoffbauer, M. A.; Klimov,V. I. Nano Lett. 2005, 5, 1039-1044.

Therefore, there remains a need for methods and compositions thatovercome these deficiencies and that effectively provide broad-emissionnanocrystals having improved quantum yield.

SUMMARY

In accordance with the purpose(s) of the invention, as embodied andbroadly described herein, the invention, in one aspect, relates toultra-small nanocrystals exhibiting a quantum yield of at least about10%.

Disclosed are inorganic nanoparticles comprising a body comprising Cdand/or Zn crystallized with selenium, sulfur, and/or tellurium; asurface covered with a multiplicity of surface ligands, whereinphosphonic acid ligands comprise at least about 20% of the total surfaceligand coverage; and a diameter of less than about 3.0 nm, wherein thenanocrystal is capable of absorbing energy from a first electromagneticregion and capable of emitting light in a second electromagnetic region,wherein the maximum absorbance wavelength of the first electromagneticregion is different from the maximum emission wavelength of the secondelectromagnetic region, thereby providing a Stokes shift of at leastabout 20 nm, wherein the second electromagnetic region comprises an atleast about 100 nm wide (FWHH) band of wavelengths, and wherein thenanoparticle exhibits has a quantum yield of at least about 10%.

Also disclosed are methods of preparing an inorganic nanoparticlecomprising the steps of heating to a temperature of greater than about300° C. a reaction mixture comprising a C₄ to C₂₂ alkyl- oraryl-phosphonic acid and a source of cadmium or zinc in a molar ratio offrom about 1:5 to about 1:1 Cd/Zn:phosphonic acid; adding to thereaction mixture an injection mixture comprising a C₂ to C₁₆ trialkyl-or triarylphosphine and a source of selenium, sulfur, or tellurium; anddecreasing the temperature of the reaction mixture to less than about300° C.

Also disclosed are the products produced by the methods of theinvention.

Also disclosed are frequency converters comprising at least one quantumdot of the invention, nanoparticle of the invention, plurality ofnanoparticles of the invention, or product of the invention, or mixturethereof dispersed within a matrix, for example, a polymeric matrix or aglass matrix.

Also disclosed are light emitting diode devices comprising a lightemitting diode capable of emitting energy of a first wavelength, and thefrequency converter of the invention positioned within an emission pathof the light emitting diode, wherein the frequency converter is capableof absorbing energy of the first wavelength.

Also disclosed are methods for producing a light emitting diode devicecomprising the step of positioning the frequency converter of theinvention within an emission path of a light emitting diode capable ofemitting energy of a first wavelength, wherein the frequency converteris capable of absorbing energy of the first wavelength.

Also disclosed are modified fluorescent light sources comprising asubstantially optically transparent and substantially hermeticallysealed tube having a first end, a second end, an interior surface, anexterior surface, and a lumen extending therethrough; a first electrodepositioned at the first end; a second electrode positioned at the secondend; inert gas vapor and mercury vapor within the lumen of the tube; aphosphor substitute comprising at least one frequency converter of theinvention, quantum dot of the invention, nanoparticle of the invention,plurality of nanoparticles of any of the invention, or product of theinvention, or mixture thereof substantially coating the interior surfaceor the exterior surface of the tube.

Also disclosed are electroluminescent devices comprising an n-typesemiconductor, a p-type semiconductor, and a quantum dot layer inelectrical or photonic communication with the n-type semiconductor andthe p-type semiconductor, wherein the quantum dot layer comprises atleast one quantum dot of the invention, nanoparticle of the invention,plurality of nanoparticles of any of the invention, product of theinvention, or frequency converter of the invention, or mixture thereof.

Also disclosed are energy cascade systems comprising at least twoquantum dots in photonic or energetic communication, wherein the firstquantum dot is capable of absorbing energy from a first absorptionelectromagnetic region and capable of emitting energy in a firstemission electromagnetic region, wherein the second quantum dot iscapable of absorbing energy from a second absorption electromagneticregion and capable of emitting energy in a second emissionelectromagnetic region, and wherein the first emission electromagneticregion overlaps with the second absorption electromagnetic region.

While aspects of the present invention can be described and claimed in aparticular statutory class, such as the system statutory class, this isfor convenience only and one of skill in the art will understand thateach aspect of the present invention can be described and claimed in anystatutory class. Unless otherwise expressly stated, it is in no wayintended that any method or aspect set forth herein be construed asrequiring that its steps be performed in a specific order. Accordingly,where a method claim does not specifically state in the claims ordescriptions that the steps are to be limited to a specific order, it isno way intended that an order be inferred, in any respect. This holdsfor any possible non-express basis for interpretation, including mattersof logic with respect to arrangement of steps or operational flow, plainmeaning derived from grammatical organization or punctuation, or thenumber or type of aspects described in the specification.

Additional advantages of the invention will be set forth in part in thedescription which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. Theadvantages of the invention will be realized and attained by means ofthe elements and combinations particularly pointed out in the appendedclaims. It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory only and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying figures, which are incorporated in and constitute apart of this specification, illustrate several embodiments and togetherwith the description serve to explain the principles of the invention.

FIG. 1 shows absorption (dashed line) and emission (solid line,λ_(ex)=367 nm) spectra of ultra-small CdSe. Static absorption andemission spectra were obtained using a Cary Bio 50 UV-Visiblespectrometer and an ISS photon counting fluorescence spectrometer,respectively.

FIG. 2 shows a C.I.E. 1931 chromaticity diagram. The chromaticitydiagram illustrates all of the colors of the visible spectrum correctedfor the response of the human eye. The dot indicates the chromaticitycoordinates of the nanoparticles of the invention. This figureillustrates that the nanoparticles produce a slightly warm white lightthat is comparable to that of a tungsten light bulb.

FIG. 3 shows white-light emission from CdSe nanoparticles of theinvention. (Left panel) Thin film of ultra-small CdSe in polyurethaneexcited by a frequency doubled titanium:sapphire laser (400 nm) withwhite-light clearly seen reflecting off the table surface. (Right panel)5 mm commercial UV LED (400 nm) illuminating a thin coating ofultra-small CdSe in polyurethane.

FIG. 4 shows a comparison of conventional tungsten and fluorescentlighting emission spectra with the AM 1.5 solar spectrum, a commercialwhite LED and ultra-small CdSe nanocrystals. The CdSe nanoparticles ofthe invention exhibit a broad emission covering most of the visiblespectrum. Unlike conventional tungsten sources, no energy is wastedproducing wavelengths beyond the visible spectrum.

FIG. 5 shows a schematic of an electroluminescent device withnanoparticles of the invention as the emitting layer.

FIG. 6 shows a schematic of the geometry for electroluminescent andfrequency converting LED device characterization.

FIG. 7 shows an absorption/emission spectrum of CdSe nanocrystalsprepared by an etching process.

FIG. 8 shows an absorption/emission spectrum of ultra-small cadmiumselenide nanocrystals having improved quantum yield.

FIG. 9 shows images of (a) three 385 nm LEDs powered at 20 mA each, andwhite-light CdSe nanocrystals in PFCB polymer at concentrations of (b)1% nanocrystals in PFCB and (c) 2% nanocrystals in PFCB, with three 385nm LEDs illuminating the samples from underneath.

FIG. 10 shows absorption spectra of PFCB, and 1%, 2%, 5%, and 10% byweight ultra-small CdSe nanocrystals dispersed in PFCB.

FIG. 11 shows emission spectra of PFCB, and 1%, 2%, 5%, and 10% byweight ultra-small CdSe nanocrystals dispersed in PFCB. The PFCB onlycurve was multiplied by ½ for scaling purposes.

FIG. 12 shows the first emission peak of ultra-small CdSe nanocrystalsplotted vs. the band-edge absorption peak. Note that below 1.7 nm theemission ceases to blue-shift, though the absorption feature doesblue-shift.

FIG. 13 shows the Michaelis-Arbuzov reaction used to synthesize variousphosphonic acids and the structures of said phosphonic acids. Phenyl andhexyl phosphonic acid are not included, as they were purchased.

FIG. 14 shows the first emission feature λ vs. band-edge absorptionfeature λ. A) Traditional and pinned nanocrystal sizes; the boxed areais the pinned region. B) Enlarged area from A showing the pinnedemission feature.

FIG. 15 shows the wavelength of pinned first emission feature vs. numberof carbons in the alkyl chain of the phosphonic acid used duringsynthesis.

FIG. 16 shows the chemical structures and the corresponding calculatedgrouped electronegativities for each structure.

FIG. 17 shows the normalized energy of pinned emission feature andelectronegativity of each phosphonic acid vs. number of carbons in thealkyl chain. The experimental and theoretical values are normalized tothe respective butyl phosphonic acid energies of each set.

FIG. 18 shows the quantum yield of dirty and clean nanocrystals: (top)ultra-small size and (bottom) traditional size regime. Clean and dirtyrefer to nanocrystal before and after the cleaning process.

DETAILED DESCRIPTION

The present invention may be understood more readily by reference to thefollowing detailed description of aspects of the invention and theExamples included therein and to the Figures and their previous andfollowing description.

Before the present compounds, compositions, articles, devices, and/ormethods are disclosed and described, it is to be understood that theyare not limited to specific synthetic methods unless otherwisespecified, or to particular reagents unless otherwise specified, as suchmay, of course, vary. It is also to be understood that the terminologyused herein is for the purpose of describing particular embodiments onlyand is not intended to be limiting. Although any methods and materialssimilar or equivalent to those described herein can be used in thepractice or testing of the present invention, example methods andmaterials are now described.

All publications mentioned herein are incorporated herein by referenceto disclose and describe the methods and/or materials in connection withwhich the publications are cited. The publications discussed herein areprovided solely for their disclosure prior to the filing date of thepresent application. Nothing herein is to be construed as an admissionthat the present invention is not entitled to antedate such publicationby virtue of prior invention. Further, the dates of publication providedherein may be different from the actual publication dates, which mayneed to be independently confirmed.

A. Definitions

As used in the specification and the appended claims, the singular forms“a,” “an” and “the” include plural referents unless the context clearlydictates otherwise. Thus, for example, reference to “a component,” “apolymer,” or “a particle” includes mixtures of two or more suchcomponents, polymers, or particles, and the like.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, another embodiment includes from the one particular valueand/or to the other particular value. Similarly, when values areexpressed as approximations, by use of the antecedent “about,” it willbe understood that the particular value forms another embodiment. Itwill be further understood that the endpoints of each of the ranges aresignificant both in relation to the other endpoint, and independently ofthe other endpoint. It is also understood that there are a number ofvalues disclosed herein, and that each value is also herein disclosed as“about” that particular value in addition to the value itself. Forexample, if the value “10” is disclosed, then “about 10” is alsodisclosed. It is also understood that when a value is disclosed that“less than or equal to” the value, “greater than or equal to the value”and possible ranges between values are also disclosed, as appropriatelyunderstood by the skilled artisan. For example, if the value “10” isdisclosed the “less than or equal to 10” as well as “greater than orequal to 10” is also disclosed. It is also understood that throughoutthe application, data is provided in a number of different formats andthat this data represents endpoints and starting points, and ranges forany combination of the data points. For example, if a particular datapoint “10” and a particular data point 15 are disclosed, it isunderstood that greater than, greater than or equal to, less than, lessthan or equal to, and equal to 10 and 15 are considered disclosed aswell as between 10 and 15. It is also understood that each unit betweentwo particular units are also disclosed. For example, if 10 and 15 aredisclosed, then 11, 12, 13, and 14 are also disclosed.

A residue of a chemical species, as used in the specification andconcluding claims, refers to the moiety that is the resulting product ofthe chemical species in a particular reaction scheme or subsequentformulation or chemical product, regardless of whether the moiety isactually obtained from the chemical species. Thus, an ethylene glycolresidue in a polyester refers to one or more —OCH₂CH₂O— units in thepolyester, regardless of whether ethylene glycol was used to prepare thepolyester. Similarly, a sebacic acid residue in a polyester refers toone or more —CO(CH₂)₈CO— moieties in the polyester, regardless ofwhether the residue is obtained by reacting sebacic acid or an esterthereof to obtain the polyester.

As used herein, the terms “optional” or “optionally” means that thesubsequently described event or circumstance may or may not occur, andthat the description includes instances where said event or circumstanceoccurs and instances where it does not.

As used herein, the term “copolymer” refers to a polymer formed from twoor more different repeating units (monomer residues). By way of exampleand without limitation, a copolymer can be an alternating copolymer, arandom copolymer, a block copolymer, or a graft copolymer.

As used herein, the term “visible spectrum” or “visible light” or“visible light spectrum” refers to the portion of the electromagneticspectrum to which the human eye is sensitive, i.e., light withwavelengths from about 400 nm to about 700 nm.

As used herein, the term “white light” refers to light energy integratedover the visible portion of the spectrum (i.e., from about 400 nm toabout 700 nm) so that all colors are blended to appear white to the eye.Such light contains approximately equal amounts of the primary additivecolors of light; the human eye perceives this light as colorless.

As used herein, the term “quantum dot” refers to a semiconductor crystalwith a nanoscale diameter, also called a nanocrystal, that because ofits small size behaves like a potential well that confines electrons inthree dimensions to a region on the order of the electrons' de Brogliewavelength in size, a few nanometers in a semiconductor. Typically, aquantum dot can absorb energy within a first electromagnetic region andemit light in a second electromagnetic region; the particular absorbanceor emission regions can depend upon the material and diameter of thequantum dot. In one aspect, the nanoparticle of the invention can beconsidered a quantum dot or nanocrystal; however, the nanoparticle ofthe invention can differ from conventional quantum dots or nanocrystalsin that the nanoparticle of the invention can emit broad band visiblelight, for example, white light.

Disclosed are the components to be used to prepare the compositions ofthe invention as well as the compositions themselves to be used withinthe methods disclosed herein. These and other materials are disclosedherein, and it is understood that when combinations, subsets,interactions, groups, etc. of these materials are disclosed that whilespecific reference of each various individual and collectivecombinations and permutation of these compounds may not be explicitlydisclosed, each is specifically contemplated and described herein. Forexample, if a particular compound is disclosed and discussed and anumber of modifications that can be made to a number of moleculesincluding the compounds are discussed, specifically contemplated is eachand every combination and permutation of the compound and themodifications that are possible unless specifically indicated to thecontrary. Thus, if a class of molecules A, B, and C are disclosed aswell as a class of molecules D, E, and F and an example of a combinationmolecule, A-D is disclosed, then even if each is not individuallyrecited each is individually and collectively contemplated meaningcombinations, A-E, A-F, B-D, B-E, B-F, C-D, C-E, and C-F are considereddisclosed. Likewise, any subset or combination of these is alsodisclosed. Thus, for example, the sub-group of A-E, B-F, and C-E wouldbe considered disclosed. This concept applies to all aspects of thisapplication including, but not limited to, steps in methods of makingand using the compositions of the invention. Thus, if there are avariety of additional steps that can be performed it is understood thateach of these additional steps can be performed with any specificembodiment or combination of embodiments of the methods of theinvention.

It is understood that the compositions disclosed herein have certainfunctions. Disclosed herein are certain structural requirements forperforming the disclosed functions, and it is understood that there area variety of structures that can perform the same function that arerelated to the disclosed structures, and that these structures willtypically achieve the same result.

B. Broad-Emission Nanocrystals

In one aspect, the invention relates to ultra-small cadmium selenide(CdSe) nanocrystals capable of emitting white light. The inorganicparticles or nanocrystals can also be referred to as quantum dots. Morespecifically, in one aspect, the invention relates to a quantum dotcapable of emitting white light. Typically, these ultra-smallnanocrystals exhibit broadband emission (e.g., from about 420 nm toabout 710 nm) throughout the visible light spectrum while not sufferingfrom self absorption due to an unusually narrow particle sizedistribution and an unusually large Stokes shift (see, e.g., FIG. 1),making these nanocrystals ideal materials for devices currently underdevelopment and also an ideal platform to study themolecule-to-nanocrystal transition. Achermann, M.; Petruska, M. A.; Kos,S.; Smith, D. L.; Koleske, D. D.; Klimov, V. I. Nature 2004, 429,642-646.

Without wishing to be bound by theory, it is believed that thisbroadband emission is the result of the size of the nanoparticle. In oneaspect, the nanoparticles of the invention are so small that the surfaceis intimately involved in the emission; that is, the nanoparticle isessentially all surface, which is not the case with larger nanocrystals.Further, without wishing to be bound by theory, it is believed that thebroadband emission is the result of the surface structure of thenanoparticle. That is, nanoparticles prepared by conventional techniquesbut of comparable size to the nanoparticles of the invention typicallydo not exhibit the broadband emission observed with the nanoparticles ofthe invention.

In a further aspect, the invention relates to an inorganic nanoparticlehaving a surface and a diameter and capable of absorbing energy from afirst electromagnetic region and capable of emitting light in a secondelectromagnetic region, wherein the diameter is less than about 5.0 nm,and wherein the second electromagnetic region comprises an at leastabout 50 nm wide (FWHH) band of wavelengths.

In a yet further aspect, the invention relates to an inorganicnanoparticle having a surface and a diameter and capable of absorbingenergy from a first electromagnetic region and capable of emitting lightin a second electromagnetic region, wherein the diameter is less thanabout 2.0 nm, and wherein the second electromagnetic region comprises anat least about 100 nm wide (FWHH) band of wavelengths.

In one aspect, the nanoparticle of the invention is not produced by anetching process. In a further aspect, the nanoparticle of the inventionis not produced by an inverse micellular process.

1. Particle Size

In one aspect, the nanoparticle of the invention has a diameter, whichcan also be referred to as a particle size. When, the nanoparticle ofthe invention is provided as a collection of nanoparticles, the diameterrefers to the average diameter of the nanoparticles in the collection.While referring to the particle size as a diameter, it is understoodthat the particles can be spherical, approximately spherical, ornonspherical. In nonspherical cases, the diameter typically refers tothe diameter of a sphere having the same hydrodynamic volume of theparticle.

In one aspect, the diameter of nanoparticle of the invention can becontrolled by the method of preparation. For example, by controlling thetemperature of the preparation mixture, growth kinetics can be favoredrelative to nucleation (also referred to as nanoparticle formation orinitiation) kinetics. Consequently, a collection of nanoparticles can beprovided with an unusually narrow particle size distribution. In afurther aspect, by further controlling the temperature of thepreparation mixture, growth kinetics can be disfavored. Consequently,the size of the nanoparticle or collection of nanoparticles can belimited to a particular size or diameter.

In one aspect, the diameter of the nanoparticle of the invention is lessthan about 3.0 nm, less than about 2.5 nm, less than about 2.0 nm, lessthan about 1.5 nm, or less than about 1.0 nm.

In a further aspect, the diameter of the nanoparticle of the inventioncan be from about 5.0 nm to about 0.5 nm, from about 5.0 nm to about 1.0nm, from about 5.0 nm to about 2.0 nm, from about 5.0 nm to about 3.0nm, from about 4.0 nm to about 0.5 nm, from about 4.0 nm to about 1.0nm, from about 4.0 nm to about 2.0 nm, from about 4.0 nm to about 3.0nm, from about 3.0 nm to about 0.5 nm, from about 3.0 nm to about 1.0nm, from about 3.0 nm to about 2.0 nm, from about 2.0 nm to about 0.5nm, or from about 2.0 nm to about 1.0 nm. In a further aspect, thediameter of the nanoparticle of the invention can be about 11 Å, about12 Å, about 13 Å, about 14 Å, about 15 Å, about 16 Å, about 17 Å, about18 Å, or about 19 Å.

2. Absorption Region

In one aspect, the nanoparticle of the invention is capable of absorbingenergy in a first electromagnetic region. That is, the nanoparticle canabsorb light having one or more wavelengths. In a further aspect, thefirst electromagnetic region comprises light with a wavelength of lessthan about 450 nm, for example less than about 425 nm, less than about400 nm, less than about 375 nm, less than about 350 nm, less than about325 nm, less than about 300 nm, less than about 275 nm, less than about250 nm, less than about 225 nm, or less than about 200 nm. In a furtheraspect, the first electromagnetic region comprises light of a wavelengthof from about 100 nm to about 450 nm, for example, from about 100 nm toabout 170 nm, from about 170 nm to about 290 nm, from about 100 nm toabout 290 nm, from about 290 nm to about 400 nm, from about 100 nm toabout 320 nm, from about 290 nm to about 320 nm, from about 320 nm toabout 400 nm, or from about 320 nm to about 450 nm. In a yet furtheraspect, the first electromagnetic region comprises light with awavelength of from about 400 nm to about 430 nm, from about 410 nm toabout 420 nm, or about 414 nm.

The absorption of a photon of light by the semiconducting material ofthe nanoparticle of the invention and subsequent emission of a lowerenergy photon by the nanoparticle of the invention results influorescence.

3. Emission Region

In one aspect, the nanoparticle of the invention is capable of emittinglight in a second electromagnetic region. That is, the nanoparticle canemit light having one or more wavelengths. When the emission comprisesmore than one wavelength, such emission comprises an emission band andcan be characterized by a maximum emission and an emission band width.Typically, absorbance band width or emission band width can be reportedas a band measurement of full-width at half-height (FWHH). For example,an emission band can comprise light of wavelengths from about 420 nm toabout 710 nm, having a maximum emission at about 560 nm and an emissionband width of about 210 nm (FWHH) (see, e.g., FIG. 1).

In one aspect, the emission band comprises one or more wavelengthswithin the second electromagnetic region. In a further aspect, theemitted wavelengths are continuous. In a yet further aspect, the emittedwavelengths are discontinuous. In a further aspect, the emission bandcomprises all wavelengths within the second electromagnetic region. In ayet further aspect, the emission band comprises less than allwavelengths within the second electromagnetic region.

In one aspect, the second electromagnetic region comprises an at leastabout 50 nm wide (FWHH) band of wavelengths, for example, at least about60 nm wide (FWHH), at least about 70 nm wide (FWHH), at least about 80nm wide (FWHH), at least about 90 nm wide (FWHH), at least about 100 nmwide (FWHH), at least about 125 nm wide (FWHH), at least about 150 nmwide (FWHH), at least about 175 nm wide (FWHH), at least about 200 nmwide (FWHH), at least about 250 nm wide (FWHH), at least about 300 nmwide (FWHH), at least about 350 nm wide (FWHH), or at least about 400 nmwide (FWHH). In a further aspect, the second electromagnetic regioncomprises an emission band at least about 100 nm wide, at least about125 nm wide, at least about 150 nm wide, at least about 175 nm wide, atleast about 200 nm wide, at least about 250 nm wide, at least about 300nm wide, at least about 350 nm wide, or at least about 400 nm wide.

In a further aspect, the second electromagnetic region can comprise aband of wavelengths at least about 50 nm wide (FWHH), at least about 60nm wide (FWHH), at least about 70 nm wide (FWHH), at least about 80 nmwide (FWHH), at least about 90 nm wide (FWHH), at least about 100 nmwide (FWHH), at least about 125 nm wide (FWHH), at least about 150 nmwide (FWHH), at least about 175 nm wide (FWHH), at least about 200 nmwide (FWHH), at least about 250 nm wide (FWHH), at least about 300 nmwide (FWHH), at least about 350 nm wide (FWHH), or at least about 400 nmwide (FWHH) in the region between about 400 nm and about 700 nm, about420 nm and about 710 nm, about 350 nm and about 750 nm, about 400 nm andabout 600 nm, about 500 nm and about 700 nm, about 300 nm and about 800nm, about 500 nm and about 750 nm, or about 350 nm and about 600 nm.

In further aspects, the second electromagnetic region can comprise lightof all wavelengths from about 400 nm to about 700 nm or the visiblelight spectrum.

4. Nanocrystal Materials

In one aspect, the nanoparticle of the invention can be provided frommaterials known to those of skill in the art of the preparation ofnanocrystals or quantum dots. For example, the nanoparticle of theinvention can comprise at least one of cadmium selenide, cadmiumsulfide, or cadmium telluride, or a mixture thereof. In another example,the nanoparticle of the invention can comprise at least one of zincsulfide, zinc selenide, zinc telluride, magnesium sulfide, magnesiumselenide, magnesium telluride, or a mixture thereof.

In a further aspect, the nanoparticle of the invention can be providedas part of a collection of nanoparticles. In such an aspect, thecollection of nanoparticles can comprise nanoparticles comprising atleast one of cadmium selenide, cadmium sulfide, cadmium telluride, zincsulfide, zinc selenide, zinc telluride, magnesium sulfide, magnesiumselenide, magnesium telluride, or a mixture thereof. The individualnanoparticles can be the same or different in composition.

5. Stokes Shift

In one aspect, absorbance region or first electromagnetic region can bedifferent from the emission region or second electromagnetic region,thereby providing a Stokes shift. Typically, the Stokes Shift isreported as the difference between a wavelength of maximum absorbanceand a wavelength of maximum emission and can be expressed in nanometers(nm). For example, a nanoparticle can have an absorbance band with amaximum absorbance of about 414 nm and an emission band with a maximumemission at about 441 nm, thereby providing a Stokes Shift of about 27nm (see, e.g., FIG. 1).

In a further aspect, the Stokes shift can be at least about 10 nm, atleast about 15 nm, at least about 20 nm, at least about 25 nm, at leastabout 30 nm, at least about 35 nm, at least about 40 nm, at least about45 nm, or at least about 50 nm. In a further aspect, the Stokes shiftcan be from about 15 nm to about 50 nm, from about 20 nm to about 50 nm,from about 30 nm to about 50 nm, from about 40 nm to about 50 nm, fromabout 20 nm to about 30 nm, from about 20 nm to about 40 nm, from about20 nm to about 50 nm, from about 30 nm to about 40 nm, or from about 30nm to about 50 nm.

6. Quantum Yield

In one aspect, the quantum dots of the invention exhibit a quantumyield. As used herein, the “quantum yield” of a radiation-inducedprocess refers to the number of times that a defined event occurs perphoton absorbed by the system. Thus, the quantum yield is a measure ofthe efficiency with which absorbed light produces some effect.

With respect to the quantum dots of the invention, quantum yieldgenerally refers to a “light in” to “light out” ratio. Such a ratio canbe measured by any of the methods for measuring electromagneticabsorbance and emission known to those of skill in the art. For example,the “light in” can be measured by a spectrophotometer set at theemission wavelength(s) of an electromagnetic energy source (e.g., anultraviolet light emitter). As another example, the “light out” can bemeasured by a spectrophotometer set at the wavelengths of white light(i.e., one or more wavelengths of visible light) emitted by the quantumdots of the invention.

The term “quantum yield” can be applied to a single quantum dot or to anaverage of a plurality of quantum dots. In theory, a quantum yield mustbe greater than 0% and is typically less than 100%. However, forphoto-induced or radiation-induced chain reactions, in which a singlephoton may trigger a long chain of transformations, quantum yieldsgreater than 100% are possible.

In one aspect, the quantum yield of the quantum dots of the inventionare greater than about 2%, for example, greater than about 3%, greaterthan about 4%, greater than about 5%, greater than about 6%, greaterthan about 7%, greater than about 8%, greater than about 9%, greaterthan about 10%, greater than about 12%, greater than about 15%, greaterthan about 20%, greater than about 25%, greater than about 30%, greaterthan about 35%, greater than about 40%, greater than about 45%, greaterthan about 50%, greater than about 55%, greater than about 60%, greaterthan about 65%, greater than about 70%, greater than about 75%, greaterthan about 80%, greater than about 85%, greater than about 90%, orgreater than about 95%.

7. Nanoparticle Surface

In one aspect, due to a very small particle size, the nanoparticles ofthe invention can have an unusually high surface area to volume ratio.Consequently, the surface composition can have an influence on theobserved properties of the nanoparticles of the invention. For example,the surface composition can be functionalized to enhance nanoparticlestability or to facilitate formation of dispersions or suspensions ofcollections of nanoparticles of the invention. Further, the surface ofthe nanocrystal can play an intimate role in the optical properties ofthe nanocrystal, such as the emission quantum yield (i.e., light in tolight out ratio) and the width of the spectrum (i.e., emission bandwidth).

The nanoparticles of the invention can be further functionalized at thesurface. Such functionalization can occur during preparation orsubsequent to preparation. In one aspect, the nanoparticle of theinvention can further comprise an inorganic shell at the surface of thenanoparticle. The inorganic shell can comprise, for example, cadmiumselenide, cadmium sulfide, cadmium telluride, zinc sulfide, zincselenide, zinc telluride, magnesium sulfide, magnesium selenide,magnesium telluride, zinc sulfide, magnesium sulfide, or a mixturethereof. In a further aspect, inorganic shell can comprise, for example,cadmium oxide, magnesium oxide, zinc oxide, aluminum oxide, titaniumdioxide, cadmium sulfoselenide, or cadmium selenium oxide. In oneaspect, the inorganic shell can be optically transparent. In a furtheraspect, the inorganic shell can be absent from the nanoparticle of theinvention.

In one aspect, the nanoparticle of the invention can further comprise anorganic ligand coating at the surface of the nanoparticle. The organicligand coating can comprise, for example, at least one of an alkylamineor arylamine residue, an alkyl- or arylphosphonic acid residue, atrialkyl- or triarylphosphine oxide residue, a trialkyl- ortriarylphosphine residue, or a mixture thereof. In a further aspect, theorganic ligand coating can comprise at least one of a hexadecylamineresidue, a dodecylphosphonic acid residue, a tri-n-octylphosphine oxideresidue, or a mixture thereof. In a yet further aspect, the organicligand coating can comprise at least one alkyl or aryl phosphine; forexample, the organic ligand coating can comprise tributylphosphine ortriphenylphosphine. In an even further aspect, the organic ligandcoating can comprise at least one alkylthiol or carboxylic acid ormixture thereof; for example, the organic ligand coating can comprise1-octanethiol or decanoic acid. In a further aspect, organic ligandcoating can be absent from the nanoparticle of the invention.

In a yet further aspect, the nanoparticle of the invention can furthercomprise an inorganic shell and an organic ligand coating at the surfaceof the nanoparticle.

8. Collections of Nanocrystals

In one aspect, the nanoparticle of the invention can be provided as partof a collection of nanoparticles comprising a plurality of nanoparticlesof the invention. In such an aspect, the plurality of nanoparticles canexhibit the absorption properties and the emission properties disclosedherein for individual nanoparticles of the invention.

For example, the plurality of nanoparticles is capable of emitting lightin a second electromagnetic region. In one aspect, the secondelectromagnetic region comprises an at least about 50 nm wide (FWHH)band of wavelengths, for example, at least about 60 nm wide (FWHH), atleast about 70 nm wide (FWHH), at least about 80 nm wide (FWHH), atleast about 90 nm wide (FWHH), at least about 100 nm wide (FWHH), atleast about 125 nm wide (FWHH), at least about 150 nm wide (FWHH), atleast about 175 nm wide (FWHH), at least about 200 nm wide (FWHH), atleast about 250 nm wide (FWHH), at least about 300 nm wide (FWHH), atleast about 350 nm wide (FWHH), or at least about 400 nm wide (FWHH). Ina further aspect, the second electromagnetic region comprises anemission band at least about 100 nm wide, at least about 125 nm wide, atleast about 150 nm wide, at least about 175 nm wide, at least about 200nm wide, at least about 250 nm wide, at least about 300 nm wide, atleast about 350 nm wide, or at least about 400 nm wide.

In a further aspect, the second electromagnetic region can comprise aband of wavelengths at least about 50 nm wide (FWHH), at least about 60nm wide (FWHH), at least about 70 nm wide (FWHH), at least about 80 nmwide (FWHH), at least about 90 nm wide (FWHH), at least about 100 nmwide (FWHH), at least about 125 nm wide (FWHH), at least about 150 nmwide (FWHH), at least about 175 nm wide (FWHH), at least about 200 nmwide (FWHH), at least about 250 nm wide (FWHH), at least about 300 nmwide (FWHH), at least about 350 nm wide (FWHH), or at least about 400 nmwide (FWHH) in the region between about 400 nm and about 700 nm, about420 nm and about 710 nm, about 350 nm and about 750 nm, about 400 nm andabout 600 nm, about 500 nm and about 700 nm, about 300 nm and about 800nm, about 500 nm and about 750 nm, or about 350 nm and about 600 nm. Infurther aspects, the second electromagnetic region can comprise light ofall wavelengths from about 400 nm to about 700 nm or the visible lightspectrum.

In one aspect, the plurality of nanoparticles of the invention can havean unusually narrow size distribution. Typically, the size distributionfor a collection of nanoparticles of the invention can be such that atleast about 80% of the nanoparticles have diameters within about 10% ofthe average diameter of the collection of nanoparticles. For example,the size distribution for a collection of nanoparticles of the inventioncan be such that at least about 50%—for example, at least about 60%, atleast about 70%, at least about 80%, at least about 90%, or at leastabout 95%—of the nanoparticles have diameters within about 50%—forexample, within about 40%, within about 30%, within about 20%, withinabout 10%, or within about 5%—of the average diameter of the collectionof nanoparticles.

C. Methods of Preparing Broad-Emission Nanocrystals

Several reported synthetic schemes have produced ultra-smallnanocrystals by growing larger nanocrystals, then etching them withvarious chemical etchants. Landes, C.; Braun, M.; Burda, C.; El-Sayed,M. A. Nano Lett. 2001, 1, 667-670; Landes, C.; El-Sayed, M. A. J. Phys.Chem. A 2002, 106, 7621-7627. Reports of ultra-small nanocrystals growninto the ultra-small, in contrast to ultra-small nanocrystals etchedfrom larger nanocrystals, are relatively rare. Qu, L.; Yu, W. W.; Peng,X. Nano Lett. 2004, 4, 465-469; Chen, X.; Samia, A. C. S.; Lou, Y.;Burda, C. J. Am. Chem. Soc. 2005, 127, 4372-4375. Peng, Z. A.; Peng, X.J. Am. Chem. Soc. 2001, 123, 183-184. Inverse micellular methods and lowtemperature organometallic preparations can also yield ultra-smallnanocrystals; however, none of these methods produce nanocrystals withcomparable optical properties to high-temperaturepyrolytically-synthesized nanocrystals. Kasuya, A.; Sivamohan, R.;Barnakov, Y. A.; Dmitruk, I. M.; Nirasawa, T.; Romanyuk, V. R.; Kumar,V.; Mamykin, S. V.; Tohji, K.; Jeyadevan, B.; Shinoda, K.; Kudo, T.;Terasaki, O.; Liu, Z.; Beloludov, R. V.; Sundararajan, V.; Kawazoe, Y.Nature Materials 2004, 3, 99-102. Rogach, A., L.; Kornowski, A.; Gao,M.; Eychmuller, A.; Weller, H. J. Phys. Chem. B 1999, 103, 3065-3069.

In one aspect, the invention relates to a method of preparing aninorganic nanoparticle comprising the steps of heating a reactionmixture comprising a C₄ to C₂₂ alkylphosphonic acid and a source ofcadmium or zinc to a temperature of greater than about 300° C.; addingto the reaction mixture an injection mixture comprising a C₂ to C₁₆trialkylphosphine and a source of selenium, sulfur, or tellurium; anddecreasing the temperature of the reaction mixture to less than about300° C. In a further aspect, the reaction mixture can further compriseat least one of a C₄ to C₂₀ trialkylphosphine oxide, or a C₈ to C₂₀alkylamine or arylamine, or a mixture thereof. In a yet further aspect,the injection mixture can further comprise a C₆ to C₂₄ hydrocarbon.

In a further aspect, the invention relates to a method of preparing aninorganic nanoparticle comprising the steps of providing anapproximately 1.0 M injection mixture comprising selenium andtri-n-butylphosphine; diluting the injection mixture with octadecene toapproximately 0.10 M concentration; providing a reaction mixturecomprising tri-n-octylphosphine oxide, hexadecylamine, cadmium oxide,and dodecylphosphonic acid; heating the reaction mixture to atemperature of about 330° C.; adding the injection mixture to thereaction mixture so as to decrease the temperature of the reactionmixture to from about 260° C. to about 270° C.; and adding toluene tothe reaction mixture so as to decrease the temperature of the reactionmixture to less than about 150° C. within about ten seconds after addingthe injection mixture to the reaction mixture.

In a further aspect, the nanocrystals of the invention are not preparedby an etching process. In a yet further aspect, the nanocrystals of theinvention are not prepared by an inverse micellular process.

1. Reaction Mixture

In one aspect, the reaction mixture comprises a C₄ to C₂₂alkylphosphonic acid and a source of cadmium or zinc. In a furtheraspect, the reaction mixture comprises a C₄ to C₂₀ trialkylphosphineoxide, a C₈ to C₂₀ alkylamine or arylamine, a C₄ to C₂₂ alkylphosphonicacid, and a source of cadmium or zinc. In one aspect, the reactionmixture comprises a solution. In a further aspect, the reaction mixturecomprises a suspension. It is understood that these components can beadded to the reaction mixture in any order. In a further aspect, one ormore of these components can be absent from the reaction mixture.

In general, the reaction mixture can be provided at any concentration.However, in one aspect, the alkylphosphonic acid is provided in molarexcess relative to the source of cadmium or zinc. For example, thealkylphosphonic acid to source of cadmium or zinc ratio can be about1:1, about 1.05:1, greater than about 1:1, about 1.5:1, about 2:1, about3:1, about 4:1, about 5:1, or greater than about 5:1.

In a further aspect, the trialkylphosphine oxide is provided relative tothe amount of the alkylamine. For example, the trialkylphosphine oxidecan present in the reaction mixture in an amount of about 0%, about 5%,about 10%, about 20%, about 30%, about 40%, about 50%, about 60%, about70%, about 80%, about 90%, about 5%, or about 100% of the total weightof the trialkylphosphine oxide and the alkylamine. Conversely, in oneaspect, the alkylamine can present in the reaction mixture in an amountof about 0%, about 5%, about 10%, about 20%, about 30%, about 40%, about50%, about 60%, about 70%, about 80%, about 90%, about 5%, or about 100%of the total weight of the trialkylphosphine oxide and the alkylamine.

In one aspect, a typical reaction mixture can comprise the components inthe following exemplary amounts: about 1 mmol of the source of cadmiumor zinc, about 2 mmol of the alkylphosphonic acid, about 18.6 mmol ofthe trialkylphosphine oxide, and about 12.25 mmol of alkylamine. It isunderstood, however, that the relative amounts of the various componentscan, of course, be varied. For example, relative to about 1 mmol of thesource of cadmium or zinc, from about 2 mmol to about 5 mmol—forexample, about 2.5 mmol, about 3 mmol, about 3.5 mmol, about 4 mmol, orabout 4.5 mmol—of the alkylphosphonic acid can be present; from about 10mmol to about 40 mmol—for example, from about 15 mmol to about 20 mmolor from about 15 mmol to about 25 mmol—of the trialkylphosphine oxidecan be present; and from about 5 mmol to about 30 mmol—for example, fromabout 10 mmol to about 15 mmol, from about 10 mmol to about 20 mmol, orfrom about 15 mmol to about 20 mmol—of alkylamine can be present.

a. Alkylamine or Arylamine

In one aspect, the reaction mixture comprises a C₈ to C₂₀ alkylamine orarylamine. The C₈ to C₂₀ alkylamine or arylamine can be any C₈ to C₂₀alkylamine or arylamine known to those of skill in the art. The C₈ toC₂₀ alkylamine or arylamine can be, for example, a C₈, C₉, C₁₀, C₁₁,C₁₂, C₁₃, C₁₄, C₁₅, C₁₆, C₁₇, C₁₈, C₁₉, or C₂₀ alkylamine or arylamine.In a further aspect, the alkylamine or arylamine can be a C₁₂ to C₂₀alkylamine or arylamine, a C₁₂ to C₁₈ alkylamine or arylamine, or a C₁₄to C₁₈ alkylamine or arylamine. In a further aspect, the alkylamine orarylamine comprises hexadecylamine. In one aspect, the alkylamine orarylamine can comprise mixtures of C₈ to C₂₀ alkylamines or arylamines.In a further aspect, the alkyl group of an alkylamine can bestraight-chain or branched and can be substituted or unsubstituted. In afurther aspect, the alkyl group of an alkylamine can be cyclic and/orthe nitrogen of the alkylamine can comprise a heteroatom of a cycliccompound, for example, pyrimidine.

In one aspect, the C₈ to C₂₀ alkylamine or arylamine can comprise aprimary amine, a secondary amine, a tertiary amine, or a mixturethereof. In one aspect, wherein the C₈ to C₂₀ alkylamine or arylamine isprovided as a secondary amine, the alkylamine can have two C₄ to C₁₀alkyl or aryl groups, which can be the same or different. In a furtheraspect, wherein the alkylamine or arylamine is provided as a secondaryamine, the alkylamine or arylamine can have two C₈ to C₂₀ alkyl or arylgroups, which can be the same or different. In one aspect, wherein theC₈ to C₂₀ alkylamine or arylamine is provided as a tertiary amine, thealkylamine or arylamine can have three C₂ to C₇ alkyl or aryl groups,which can be the same or different and total to 8 to 20 carbon units. Ina further aspect, wherein the alkylamine or arylamine is provided as atertiary amine, the alkylamine can have three C₈ to C₂₀ alkyl or arylgroups, which can be the same or different.

In one aspect, the C₈ to C₂₀ alkylamine or arylamine can comprise anaromatic amine. That is, the nitrogen of the amine can be bound to oneor more aromatic moieties, for example, to one or more benzene,naphthalene, pyridine, imidazole, chlorobenzene, toluene, or anilineresidues. In a further aspect, the aryl group(s) of an arylamine can bethe same or different and can be substituted or unsubstituted. In afurther aspect, the nitrogen of the arylamine can comprise a heteroatomof a cyclic aromatic compound, for example, pyridine.

It is also understood that the C₈ to C₂₀ alkylamine or arylamine can beabsent from the reaction mixture.

b. Alkyl- or Arylphosphonic Acid

In one aspect, the reaction mixture comprises a C₄ to C₂₂ alkyl- orarylphosphonic acid. The C₄ to C₂₂ alkyl- or arylphosphonic acid can beany C₄ to C₂₂ alkyl- or arylphosphonic acid known to those of skill inthe art. The C₄ to C₂₂ alkyl- or arylphosphonic acid can be, forexample, a C₄, C₅, C₆, C₇, C₈, C₉, C₁₀, C₁₁, C₁₂, C_(D), C₁₄, C₁₅, C₁₆,C₁₇, C₁₈, C₁₉, C₂₀, C₂₁, or C₂₂ alkyl- or arylphosphonic acid. In afurther aspect, the alkyl- or arylphosphonic acid can be a C₄ to C₁₆alkyl- or arylphosphonic acid, or a C₁₀ to C₁₄ alkyl- or arylphosphonicacid. In a further aspect, the alkyl- or arylphosphonic acid comprisesdodecylphosphonic acid. In one aspect, the alkyl- or arylphosphonic acidcan comprise mixtures of C₄ to C₂₂ alkyl- or arylphosphonic acids. In afurther aspect, the alkyl group of the alkyl- or arylphosphonic acid canbe straight-chain or branched and can be substituted or unsubstituted.

In a further aspect, an alkyl group of an alkyl- or arylphosphonic acidcan be straight-chain or branched and can be substituted orunsubstituted. In a further aspect, an alkyl group of an alkyl- orarylphosphonic acid can be cyclic and/or the phosphorous of an alkyl- orarylphosphonic acid can comprise a heteroatom of a cyclic compound.

In one aspect, an alkyl- or arylphosphonic acid can comprise an aromaticalkyl- or arylphosphonic acid. That is, the phosphorous of thephosphonic acid can be bound to an aromatic moiety, for example, to abenzene, a naphthalene, a pyridine, an imidazole, a toluene, or ananiline residue. In a further aspect, the aryl group(s) of an alkyl- orarylphosphonic acid can be substituted or unsubstituted. In a furtheraspect, the phosphorous of the alkyl- or arylphosphonic acid cancomprise a heteroatom of a cyclic aromatic compound.

In one aspect, alkyl- or aryl-phosphonic acid moieties comprise at leastabout 20% of the total surface ligand coverage at the nanoparticlesurface. For example, alkyl- or aryl-phosphonic acid moieties cancomprise at least about 25%, at least about 30%, at least about 35%, atleast about 40%, at least about 45%, at least about 50%, at least about55%, at least about 60%, at least about 65%, at least about 70%, atleast about 75%, at least about 80%, at least about 85%, at least about90%, at least about 95%, or substantially all of the total surfaceligand coverage.

In a further aspect, the C₄ to C₂₂ alkyl- or aryl-phosphonic acid andthe source of cadmium or zinc can be provided in the reaction mixture ina molar ratio of from about 1:5 to about 1:1 Cd/Zn:phosphonic acid. Forexample, the C₄ to C₂₂ alkyl- or aryl-phosphonic acid and the source ofcadmium or zinc can be provided in a molar ratio of from about 1:5 toabout 1:1, of from about 1:4.5 to about 1:1, of from about 1:4 to about1:1, of from about 1:3.5 to about 1:1, of from about 1:3 to about 1:1,of from about 1:2.5 to about 1:1, of from about 1:2 to about 1:1, offrom about 1: to about 1:1, of from about 1:5 to about 1:1.5, of fromabout 1:5 to about 1:2, of from about 1:4.5 to about 1:1.5, of fromabout 1:4 to about 1:2, of from about 1:3.5 to about 1:2, of from about1:3 to about 1:2, of from about 1:2.5 to about 1:2, of from about 1:2.5to about 1:1.5, of about 1:5, of about 1:4.5, of about 1:4, of about1:3.5, of about 1:3, of about 1:2.5, of about 1:2, of about 1:1.5, ofabout 1:1 Cd/Zn:phosphonic acid.

c. Trialkyl- or Triarylphosphine Oxide

In one aspect, the reaction mixture comprises a C₄ to C₂₀trialkylphosphine oxide. The C₄ to C₂₀ trialkylphosphine oxide can beany C₄ to C₂₀ trialkylphosphine oxide known to those of skill in theart. That is, each alkyl group of the trialkylphosphine oxide cancomprise a C₄ to C₂₀ alkyl group. The C₄ to C₂₀ trialkylphosphine oxidecan be, for example, a C₄, C₅, C₆, C₇, C₈, C₉, C₁₀, C₁₁, C₁₂, C₁₃, C₁₄,C₁₅, C₁₆, C₁₇, C₁₈, C₁₉, or C₂₀ trialkylphosphine oxide. In a furtheraspect, the trialkylphosphine oxide can be a C₄ to C₁₂ trialkylphosphineoxide or a C₆ to C₁₀ trialkylphosphine oxide. In a further aspect, theC₄ to C₂₀ trialkylphosphine oxide comprises tri-n-octylphosphine oxide.In one aspect, the trialkylphosphine oxide can comprise mixtures of C₈to C₂₀ trialkylphosphine oxides. In a further aspect, the alkyl groupsof the trialkylphosphine oxide can be straight-chain or branched, can besubstituted or unsubstituted, and can be the same or different.

In a further aspect, one or more alkyl group(s) of an trialkyl- ortriarylphosphine oxide can be straight-chain or branched and can besubstituted or unsubstituted. In a further aspect, one or more alkylgroup(s) of an trialkyl- or triarylphosphine oxide can be cyclic and/orthe phosphorous of an trialkyl- or triarylphosphine oxide can comprise aheteroatom of a cyclic compound.

In one aspect, an trialkyl- or triarylphosphine oxide can comprise anaromatic trialkyl- or triarylphosphine oxide. That is, the phosphorousof the trialkyl- or triarylphosphine oxide can be bound to one or morearomatic moieties, for example, to benzene, naphthalene, pyridine,imidazole, chlorobenzene, toluene, or aniline residues. In a furtheraspect, the aryl group(s) of an alkyl- or arylphosphonic acid can be thesame or different and can be substituted or unsubstituted.

It is also understood that the C₄ to C₂₀ trialkylphosphine oxide can beabsent from the reaction mixture.

d. Source of Cadmium or Zinc

In one aspect, the reaction mixture comprises a source of cadmium orzinc. That is, in one aspect, the reaction mixture comprises at leastone compound capable of providing cadmium, zinc, or a mixture thereof.In a further aspect, the reaction mixture comprises more than onecompound capable of providing cadmium, zinc, or a mixture thereof.

The source of cadmium or zinc can be any source of cadmium or zinc knownto those of skill in the art. For example, the source of cadmium or zinccan comprise at least one of an oxide of cadmium or zinc, a dialkylcadmium or dialkyl zinc, a carboxylate salt of cadmium or zinc or aphosphonate salt of cadmium or zinc or a mixture thereof.

In one aspect, the source of cadmium or zinc can comprise at least oneof an oxide of cadmium or zinc. That is, the source of cadmium or zinccan be, for example, at least one of zinc oxide or cadmium oxide or amixture thereof.

In one aspect, the source of cadmium or zinc can comprise at least oneof a dialkyl cadmium or dialkyl zinc or a mixture thereof. In oneaspect, the alkyl groups can be the same or different, can besubstituted or unsubstituted, and can be straight-chain or branched. Ina further aspect, the alkyl can be one or more C₁ to C₆ alkyl groups,for example, at least one of methyl, ethyl, propyl, butyl, pentyl, orhexyl groups or a mixture thereof. In further aspects, the alkyl can beone or more C₁ to C₁₂ alkyl groups or C₆ to C₁₂ alkyl groups.

In one aspect, the source of cadmium or zinc can comprise at least oneof a carboxylate salt of cadmium or zinc or a phosphonate salt ofcadmium or zinc or a mixture thereof. In one aspect, the carboxylate ora phosphonate group can be alkyl or aryl, can be substituted orunsubstituted, and can be straight-chain or branched. In a furtheraspect, the carboxylate or phosphonate group can be one or more C₁ to C₆carboxylate or phosphonate groups, for example, at least one of methyl,ethyl, propyl, butyl, pentyl, or hexyl carboxylate or phosphonate groupor a mixture thereof. In further aspects, the carboxylate or phosphonategroup can be one or more C₁ to C₁₂ carboxylate or phosphonate groups orC₆ to C₁₂ carboxylate or phosphonate groups.

In a yet further aspect, the source of cadmium or zinc comprises atleast one of zinc oxide, dimethyl zinc, cadmium oxide, cadmium acetate,cadmium stearate, or dimethyl cadmium or a mixture thereof. In a furtheraspect, the source of cadmium or zinc comprises cadmium oxide.

In one aspect, the source of cadmium or zinc and the alkylphosphonicacid can be provided in a molar ratio of from about 5:1 Cd/Zn:phosphonicacid to about 1:1 Cd/Zn:phosphonic acid. For example, the source ofcadmium or zinc and the alkylphosphonic acid can be provided in a molarratio of from about 4.5:1 Cd/Zn:phosphonic acid, about 4:1Cd/Zn:phosphonic acid, about 3.5:1 Cd/Zn:phosphonic acid, about 3:1Cd/Zn:phosphonic acid, about 2.5:1 Cd/Zn:phosphonic acid, about 2:1Cd/Zn:phosphonic acid, or about 1.5:1 Cd/Zn:phosphonic acid. In afurther aspect, the source of cadmium or zinc and the alkylphosphonicacid can be provided in a molar ratio of greater than about 5:1Cd/Zn:phosphonic acid.

In another aspect, the source of cadmium or zinc and the alkylphosphonicacid can be provided in a molar ratio of from about 1:5 Cd/Zn:phosphonicacid to about 1:1 Cd/Zn:phosphonic acid. For example, the source ofcadmium or zinc and the alkylphosphonic acid can be provided in a molarratio of from about 1:4.5 Cd/Zn:phosphonic acid, about 1:4Cd/Zn:phosphonic acid, about 1:3.5 Cd/Zn:phosphonic acid, about 1:3Cd/Zn:phosphonic acid, about 1:2.5 Cd/Zn:phosphonic acid, about 1:2Cd/Zn:phosphonic acid, or about 1:1.5 Cd/Zn:phosphonic acid. In afurther aspect, the source of cadmium or zinc and the alkylphosphonicacid can be provided in a molar ratio of greater than about 1:5Cd/Zn:phosphonic acid. In still other aspects, the molar ratio of thesource of cadmium or zinc and the phosphonic acid can be less than orgreater than any values specifically recited herein, and the presentinvention is not intended to be limited to any particular ratio.

e. Reaction Temperature

In one aspect, the reaction mixture is heated to a temperature beforethe injection mixture is added. In a further aspect, the reactionmixture is heated to a temperature sufficient to facilitate bothinitiation of nanocrystal formation and growth of nanocrystals beforethe injection mixture is added.

For example, the temperature can be greater than about 200° C., greaterthan about 225° C., greater than about 250° C., greater than about 275°C., greater than about 300° C., greater than about 310° C., greater thanabout 320° C., greater than about 330° C., greater than about 340° C.,or greater than about 350° C. In a further aspect, the temperature canbe less than about 400° C., less than about 450° C., less than about400° C., less than about 375° C., less than about 350° C., less thanabout 325° C., less than about 300° C., less than about 275° C., lessthan about 250° C., or less than about 225° C.

In a further aspect, the reaction mixture can be heated to a temperatureof from about 200° C. to about 400° C. before the injection mixture isadded. For example, the temperature can be from about 250° C. to about400° C., from about 300° C. to about 400° C., from about 250° C. toabout 350° C., from about 300° C. to about 350° C., about 310° C., about320° C., about 330° C., or about 340° C.

2. Injection Mixture

In one aspect, the injection mixture comprises a C₂ to C₁₆ trialkyl- ortriarylphosphine and a source of selenium, sulfur, or tellurium. In afurther aspect, the injection mixture comprises a C₂ to C₁₆ trialkyl- ortriarylphosphine, a C₆ to C₂₄ hydrocarbon, and a source of selenium,sulfur, or tellurium. In one aspect, the injection mixture comprises asolution. In a further aspect, the injection mixture comprises asuspension. It is understood that these components can be added to theinjection mixture in any order. In a further aspect, one or more ofthese components can be absent from the injection mixture.

In general, the injection mixture can be provided at any concentration.However, in one aspect, the injection mixture can be provided as asolution of the source of selenium, sulfur, or tellurium in the C₂ toC₁₆ trialkyl- or triarylphosphine, which can then be diluted with a C₆to C₂₄ hydrocarbon. In a further aspect, the injection mixture can beprovided as a suspension of the source of selenium, sulfur, or telluriumin the C₂ to C₁₆ trialkyl- or triarylphosphine, which can then bediluted with a C₆ to C₂₄ hydrocarbon. The mixture can be provided at anydesired concentration and is typically provided at a concentration offrom about 0.1 M to about 7 M. For example, the source of selenium,sulfur, or tellurium can be provided as an approximately 1.0 M (e.g.,approximately 0.25 M, approximately 0.5 M, approximately 0.75 M,approximately 1.25 M, approximately 1.5 M, approximately 1.75 M,approximately 2 M, approximately 2.5 M, or approximately 3 M) mixture inthe C₂ to C₁₆ trialkyl- or triarylphosphine and can then be optionallydiluted with a C₆ to C₂₄ hydrocarbon to, for example, an approximately0.1 M (e.g., approximately 0.025 M, approximately 0.05 M, approximately0.075 M, approximately 0.125 M, approximately 0.15 M, approximately0.175 M, approximately 0.2 M, approximately 0.25 M, or approximately 0.3M) concentration.

In one aspect, source of selenium, sulfur, or tellurium can be providedin an amount and concentration so as to achieve a Cd/Zn:Se/S/Te ratio ofgreater than about 1.01:1 after the injection mixture is added to thereaction mixture. That is, in one aspect, the overall reaction can bemaintained cation-rich (i.e., Cd- or Zn-rich). For example, theCd/Zn:Se/S/Te ratio can be greater than about 1.01:1, greater than about1.05:1, greater than about 1.1:1, greater than about 1.2:1, greater thanabout 1.3:1, greater than about 1.4:1, greater than about 1.5:1, greaterthan about 2:1, greater than about 3:1, greater than about 4:1, orgreater than about 5:1.

In a further aspect, the Cd/Zn:Se/S/Te ratio can be from about 1.01:1 toabout 5:1, for example, about 1.05:1 to about 4:1, about 1.1:1 to about3:1, about 1.5:1 to about 2:1, about 1.5:1 to about 2.5:1, about 2:1 toabout 2.5:1, about 1.01:1, about 1.05:1, about 1.1:1, about 1.2:1, about1.3:1, about 1.4:1, about 1.5:1, about 2:1, about 3:1, greater thanabout 4:1, or about 5:1.

In a yet further aspect, the Cd/Zn:Se/S/Te ratio can be from about1:1.01 to about 1:5, for example, about 1:1.05 to about 1:4, about 1:1.1to about 1:3, about 1:1.5 to about 1:2, about 1:1.5 to about 1:2.5,about 1:2 to about 1:2.5, about 1:1.01, about 1:1.05, about 1:1.1, about1:1.2, about 1:1.3, about 1:1.4, about 1:1.5, about 1:2, about 1:3,greater than about 1:4, or about 1:5. In a still further aspect, theCd/Zn:Se/S/Te ratio can be about 1:1.3, for example, from about 1:1.0 toabout 1:6, from about 1:1.1 to about 1:5, from about 1:1.2 to about 1:4,or from about 1:1.25 to about 1:35.

In a further aspect, the C₂ to C₁₆ trialkyl- or triarylphosphine isprovided relative to the amount of the C₆ to C₂₄ hydrocarbon. Forexample, the trialkyl- or triarylphosphine can present in the reactionmixture in an amount of about 1%, about 2.5%, about 5%, about 10%, about15%, about 20%, about 30%, about 40%, about 50%, about 60%, about 70%,about 80%, about 90%, about 95%, or about 100%, of the total weight ofthe trialkyl- or triarylphosphine and the hydrocarbon. Conversely, inone aspect, the hydrocarbon can present in the reaction mixture in anamount of about 99%, about 97.5%, about 95%, about 90%, about 85%, about80%, about 70%, about 60%, about 50%, about 40%, about 30%, about 20%,about 10%, about 5%, or about 0% of the total weight of trialkyl- ortriarylphosphine and the hydrocarbon. It is understood that the relativeamounts can also be expressed as a ratio.

In certain aspects, the C₆ to C₂₄ hydrocarbon and the C₂ to C₁₆trialkyl- or triarylphosphine are provided in a ratio in the injectionsolution of less than about 70:30, of about 70:30, of less than about75:25, or of about 75:25. In further aspects, the C₆ to C₂₄ hydrocarboncan be octadecene, and the C₂ to C₁₆ trialkyl- or triarylphosphine canbe tri-n-butylphosphine. For example, the C₆ to C₂₄ hydrocarbon can beoctadecene, the C₂ to C₁₆ trialkyl- or triarylphosphine can betri-n-butylphosphine are, and the octadecene and thetri-n-butylphosphine can be provided in a ratio in the injectionsolution of from about 80:20 to about 60:40. In a further example, theC₆ to C₂₄ hydrocarbon can be octadecene, the C₂ to C₁₆ trialkyl- ortriarylphosphine can be tri-n-butylphosphine are, and the octadecene andthe tri-n-butylphosphine can be provided in a ratio in the injectionsolution of from about 70:30 to about 75:25. In certain aspects, thesemethods can prepare inorganic nanoparticles having a quantum yield of atleast about 5%, at least about 8%, or at least about 10%.

Without wishing to be bound by theory, it is believed that the ratio ofthe C₂ to C₁₆ trialkyl- or triarylphosphine to the C₆ to C₂₄ hydrocarboncan affect the quantum yield of the resultant nanocrystals. That is,certain ratios of the C₂ to C₁₆ trialkyl- or triarylphosphine to the C₆to C₂₄ hydrocarbon can increase the rate of reduction of cadmium oxideor zinc oxide, thereby facilitating availability of the metal. Further,without wishing to be bound by theory, it is believed that the disclosedratios of the C₂ to C₁₆ trialkyl- or triarylphosphine to the C₆ to C₂₄hydrocarbon can facilitate formation of the core crystal structure ofthe nanocrystals, thereby providing superior structures.

a. Trialkyl- or Triarylphosphine

In one aspect, the reaction mixture comprises a C₂ to C₁₆ trialkyl- ortriarylphosphine. The C₂ to C₁₆ trialkyl- or triarylphosphine can be anyC₂ to C₁₆ trialkyl- or triarylphosphine known to those of skill in theart. That is, each alkyl group of a C₂ to C₁₆ trialkylphosphine cancomprise a C₂ to C₁₆ alkyl group. A C₂ to C₁₆ trialkylphosphine can be,for example, a C₂, C₃, C₄, C₅, C₆, C₇, C₈, C₉, C₁₀, C₁₁, C₁₂, C₁₃, C₁₄,C₁₅, or C₁₆ trialkylphosphine. In a further aspect, a trialkylphosphinecan be a C₂ to C₁₂ trialkylphosphine, a C₂ to C₁₀ trialkylphosphine, aC₂ to C₈ trialkylphosphine, or a C₂ to C₆ trialkylphosphine. In afurther aspect, a C₂ to C₁₆ trialkylphosphine comprisestri-n-butylphosphine. In one aspect, a C₂ to C₁₆ trialkylphosphine cancomprise mixtures of C₂ to C₁₆ trialkylphosphines. In a further aspect,the alkyl groups of a C₂ to C₁₆ trialkylphosphine can be straight-chainor branched, can be substituted or unsubstituted, and can be the same ordifferent. In a further aspect, the alkyl group of a trialkylphosphinecan be cyclic and/or the phosphorous of a trialkylphosphine can comprisea heteroatom of a heterocyclic compound.

In one aspect, the C₂ to C₁₆ trialkyl- or triarylphosphine can comprisean aromatic phosphine. That is, the phosphorous of the phosphine can bebound to one or more aromatic moieties, for example, to one or morebenzene, naphthalene, pyridine, imidazole, chlorobenzene, toluene, oraniline residues. In a further aspect, the aryl group(s) of anarylphosphinecan be the same or different and can be substituted orunsubstituted. In a further aspect, the phosphorous of the arylphosphinecan comprise a heteroatom of a cyclic aromatic compound.

It is also understood that the C₂ to C₁₆ trialkyl- or triarylphosphinecan be absent from the reaction mixture.

b. Hydrocarbon

In one aspect, the reaction mixture comprises a C₆ to C₂₄ hydrocarbon.The hydrocarbon can be any C₆ to C₂₄ hydrocarbon known to those of skillin the art. The C₆ to C₂₄ hydrocarbon can be, for example, a C₆, C₇, C₈,C₉, C₁₀ C₁₁, C₁₂, C₁₃, C₁₄, C₁₅, C₁₆ C₁₇, C₁₈, C₁₉, C₂₀, C₂₁, C₂₂, C₂₃,or C₂₄ hydrocarbon. In a further aspect, the C₆ to C₂₄ hydrocarbon canbe a C₆ to C₁₂ hydrocarbon, a C₁₄ to C₂₂ hydrocarbon, or a C₁₆ to C₂₀hydrocarbon. In a further aspect, the C₆ to C₂₄ hydrocarbon comprisesoctadecene. In one aspect, the C₆ to C₂₄ hydrocarbon can comprisemixtures of C₆ to C₂₄ hydrocarbons. In a further aspect, the C₆ to C₂₄hydrocarbon can be straight-chain or branched and can be substituted orunsubstituted. For example, halogenated hydrocarbons can be used. In afurther aspect, the C₆ to C₂₄ hydrocarbon can comprise a cyclichydrocarbon.

In a further aspect, the C₆ to C₂₄ hydrocarbon can comprise an aromatichydrocarbon. That is, the hydrocarbon can comprise, for example,benzene, naphthalene, pyridine, imidazole, chlorobenzene, toluene, oraniline. Further, in one aspect, the hydrocarbon can comprise one ormore benzene, naphthalene, pyridine, imidazole, chlorobenzene, toluene,or aniline residues. In a further aspect, the aryl group(s) of the C₆ toC₂₄ hydrocarbon can be the same or different and can be substituted orunsubstituted.

It is also understood that the C₆ to C₂₄ hydrocarbon can be absent fromthe reaction mixture.

c. Source of Selenium, Sulfur, or Tellurium

In one aspect, the injection mixture comprises a source of selenium,sulfur, or tellurium. That is, in one aspect, the injection mixturecomprises at least one compound capable of providing selenium, sulfur,or tellurium or a mixture thereof. In a further aspect, the reactionmixture comprises more than one compound capable of providing selenium,sulfur, or tellurium or a mixture thereof.

The source of selenium, sulfur, or tellurium can be any source ofselenium, sulfur, or tellurium known to those of skill in the art. Inone aspect, the source of selenium, sulfur, or tellurium comprises atleast one of selenium powder, sulfur powder, or tellurium powder or amixture thereof. In a further aspect, the source of selenium, sulfur, ortellurium comprises at least one of hexamethyl silylsulfide, hexamethylsilylselenide, or hexamethyl silyltelluride or a mixture thereof.

d. Injection Temperature

In one aspect, the injection mixture can be provided at any convenienttemperature. In a further aspect, the injection mixture can be providedat a temperature lower than that of the reaction mixture temperature.For example, the injection mixture can be provided at room temperature.It is also understood that the injection mixture can be provided at atemperature higher than room temperature. Typically, the injectionmixture is provided at a temperature of equal to or less than thatnecessary to maintain nanocrystal growth. In one aspect, the injectionmixture is provided at a temperature of less than that necessary tomaintain nanocrystal formation initiation. By “the injection mixture canbe provided,” it is meant that, in one aspect, the injection mixture canbe added to the reaction mixture at the provided temperature.

In one aspect, the temperature of the injection mixture can bemaintained at a temperature of less than that necessary to initiatenanocrystal formation. That is, an injection mixture containing both asource of source of cadmium or zinc and a source of selenium, sulfur, ortellurium can be provided at a temperature of less than that necessaryto initiate nanocrystal formation until a heating step is performed,thereby initiating nanocrystal formation. In a further aspect, thetemperature of the injection mixture can be maintained at a temperatureof less than that necessary to maintain nanocrystal growth. That is, aninjection mixture containing a source of selenium, sulfur, or telluriumcan be provided at a temperature of less than that necessary to initiatenanocrystal formation until added to the reaction mixture, therebydecreasing the temperature of the reaction mixture to less than thatnecessary to initiate nanocrystal formation.

In a further aspect, the injection mixture can be provided at less thanabout 300° C., less than about 275° C., less than about 250° C., lessthan about 225° C., less than about 200° C., less than about 175° C.,less than about 150° C., less than about 125° C., less than about 100°C., less than about 75° C., or less than about 50° C. In a furtheraspect, the injection mixture can be provided at about 300° C., about275° C., about 250° C., about 225° C., about 200° C., about 175° C.,about 150° C., about 125° C., about 100° C., about 75° C., about 50° C.,or about 25° C.

In a further aspect, the injection mixture can be provided at atemperature of from about 25° C. to about 300° C., from about 25° C. toabout 275° C., from about 25° C. to about 250° C., from about 25° C. toabout 225° C., from about 25° C. to about 250° C., from about 25° C. toabout 225° C., from about 25° C. to about 200° C., from about 25° C. toabout 175° C., from about 25° C. to about 150° C., from about 25° C. toabout 125° C., from about 25° C. to about 100° C., from about 200° C. toabout 300° C., from about 100° C. to about 200° C., from about 150° C.to about 250° C., or from about 50° C. to about 150° C.

In a yet further aspect, the injection mixture can be provided attemperature less than room temperature. For example, the injectionmixture can be provided at a temperature of from about 0° C. to about25° C., from about 5° C. to about 25° C., from about 10° C. to about 25°C., from about 15° C. to about 25° C., from about 0° C. to about 10° C.,or of less than about 0° C.

3. Temperature Decrease

In one aspect, the injection mixture can be added to the reactionmixture so as to decrease the temperature of the reaction mixture and,thus, the temperature of the resulting combined mixture. In one aspect,the addition of the injection mixture lowers the temperature of thecombined mixture to a temperature at which nanocrystal initiation isdisfavored, yet nanocrystal growth is still favored. In a furtheraspect, the addition of the injection mixture lowers the temperature ofthe combined mixture to a temperature at which both nanocrystalinitiation and nanocrystal growth are disfavored.

In one aspect, addition of the injection mixture decreases thetemperature of the reaction mixture to less than about 300° C., forexample, to less than about 270° C., to less than about 250° C., to lessthan about 225° C., to less than about 200° C., to less than about 175°C., to less than about 150° C., or to less than about 125° C. In afurther aspect, addition of the injection mixture decreases thetemperature of the reaction mixture to from about 150° C. to about 300°C., from about 150° C. to about 250° C., from about 150° C. to about200° C., from about 200° C. to about 250° C., or from about 250° C. toabout 300° C.

It is understood that both injection mixture temperature and injectionmixture addition rate can influence the temperature of the combinedmixture and, thus the kinetics of nanocrystal initiation and nanocrystalgrowth. Typically, nucleation can occur at relatively highertemperatures, and growth can occur at relatively lower temperatures. If,for example, injection is distributed over a longer time period, thetemperature remains at the nucleation temperature longer and, thus, morenanocrystals can nucleate. If, as a further example, injection isdistributed over a shorter time period, the temperature remains at thenucleation temperature for less time and, thus, fewer nanocrystals cannucleate.

In one aspect, the temperature of the reaction mixture is decreased toless than a temperature within a time after adding the injection mixtureto the reaction mixture. For example, the temperature can be about 330°C., about 320° C., about 310° C., about 300° C., about 290° C., about280° C., about 270° C., about 260° C., about 250° C., about 240° C.,about 230° C., about 220° C., about 210° C., about 200° C., about 190°C., about 180° C., about 170° C., about 160° C., or about 150° C. Forexample, the time can be about 90 seconds, about 60 seconds, about 30seconds, about 20 seconds, about 10 seconds, or about 5 seconds.

It is also understood that external cooling can be used to decrease thetemperature of the combined mixture, both in place of and in concertwith the cooling effect of the addition of injection mixture.

4. Temperature Control Solvents

In one aspect, a solvent can be added to the combined mixture afteraddition of the injection mixture, or simultaneous with the addition ofthe injection mixture, so as to decrease the temperature of the combinedmixture. That is, the addition of a solvent can lower the temperature ofthe combined mixture. In one aspect, the addition of a solvent mixturelowers the temperature of the combined mixture to a temperature at whichboth nanocrystal initiation and nanocrystal growth are disfavored.

In one aspect, a solvent can be provided at any convenient temperature.In a further aspect, a solvent can be provided at a temperature lowerthan that of the combined mixture temperature. For example, a solventcan be provided at room temperature. Typically, a solvent is provided ata temperature of less than that necessary to maintain nanocrystalgrowth. By “a solvent can be provided,” it is meant that, in one aspect,a solvent can be added to the combined mixture at the providedtemperature.

In a further aspect, a solvent can be provided at less than about 300°C., less than about 275° C., less than about 250° C., less than about225° C., less than about 200° C., less than about 175° C., less thanabout 150° C., less than about 125° C., less than about 100° C., lessthan about 75° C., or less than about 50° C. In a further aspect, asolvent can be provided at about 300° C., about 275° C., about 250° C.,about 225° C., about 200° C., about 175° C., about 150° C., about 125°C., about 100° C., about 75° C., about 50° C., or about 25° C.

In a further aspect, a solvent can be provided at a temperature of fromabout 25° C. to about 300° C., from about 25° C. to about 275° C., fromabout 25° C. to about 250° C., from about 25° C. to about 225° C., fromabout 25° C. to about 250° C., from about 25° C. to about 225° C., fromabout 25° C. to about 200° C., from about 25° C. to about 175° C., fromabout 25° C. to about 150° C., from about 25° C. to about 125° C., fromabout 25° C. to about 100° C., from about 200° C. to about 300° C., fromabout 100° C. to about 200° C., from about 150° C. to about 250° C., orfrom about 50° C. to about 150° C.

In a yet further aspect, a solvent can be provided at temperature lessthan room temperature. For example, a solvent can be provided at atemperature of from about 0° C. to about 25° C., from about 5° C. toabout 25° C., from about 10° C. to about 25° C., from about 15° C. toabout 25° C., from about 0° C. to about 10° C., or of less than about 0°C.

In one aspect, addition of the injection mixture decreases thetemperature of the reaction mixture to less than about 300° C., forexample, to less than about 270° C., to less than about 250° C., to lessthan about 225° C., to less than about 200° C., to less than about 175°C., to less than about 150° C., or to less than about 125° C. In afurther aspect, addition of the injection mixture decreases thetemperature of the reaction mixture to from about 150° C. to about 300°C., from about 150° C. to about 250° C., from about 150° C. to about200° C., from about 200° C. to about 250° C., or from about 250° C. toabout 300° C.

It is understood that both solvent temperature and solvent addition ratecan influence the temperature of the combined mixture and, thus thekinetics of nanocrystal initiation and nanocrystal growth. In oneaspect, the temperature of the combined mixture is decreased to lessthan a temperature within a time after adding the solvent to thecombined mixture. For example, the temperature can be about 330° C.,about 320° C., about 310° C., about 300° C., about 290° C., about 280°C., about 270° C., about 260° C., about 250° C., about 240° C., about230° C., about 220° C., about 210° C., about 200° C., about 190° C.,about 180° C., about 170° C., about 160° C., or about 150° C. Forexample, the time can be about 90 seconds, about 60 seconds, about 30seconds, about 20 seconds, about 10 seconds, or about 5 seconds.

It is also understood that external cooling can be used to decrease thetemperature of the combined mixture, both in place of and in concertwith the cooling effect of the addition of a solvent.

In one aspect, the solvent comprises at least one C₁ to C₁₂ alcohol. Forexample, the alcohol can be methanol, ethanol, propanol, or butanol. Thealcohol can be straight-chain or branched and can be substituted orunsubstituted. In a further aspect, mixtures of alcohols can be used. Asa further example, the C₁ to C₁₂ alcohol can be a diol or a triol. In afurther aspect, the solvent comprises at least one C₅ to C₁₂ hydrocarbonor C₅ to C₁₂ cyclic hydrocarbon or a mixture thereof. The hydrocarboncan be straight-chain or branched and can be substituted orunsubstituted. In a further aspect, mixtures of hydrocarbons can beused. In a further example, the solvent can be an aromatic solvent. Forexample, the solvent can be at least one of benzene, toluene, or xyleneor a mixture thereof. As a further example, halogenated solvents can beused. In a further aspect, toluene can be added to the reaction mixtureso as to decrease the temperature of the reaction mixture to less thanabout 150° C.

It is also understood that the use of temperature control solvents canbe absent from the invention.

5. Combined Injection Method

In one aspect, the invention relates to a method of preparing aninorganic nanoparticle comprising the sequential steps of heating acomposition comprising a source of cadmium or zinc and a source ofselenium, sulfur, or tellurium to a temperature sufficient to initiatenanoparticle formation and sufficient to support nanoparticle growth;cooling the composition to a temperature insufficient to initiatenanoparticle formation but sufficient to support nanoparticle growth;and cooling the composition to a temperature insufficient to initiatenanoparticle formation and insufficient to support nanoparticle growth.In a further aspect, the composition can further comprise at least oneof a C₄ to C₂₀ trialkyl- or triarylphosphine oxide, a C₈ to C₂₀alkylamine or arylamine, a C₄ to C₂₂ alkyl- or arylphosphonic acid, a C₂to C₁₆ trialkyl- or triarylphosphine, or a C₆ to C₂₄ hydrocarbon or amixture thereof. It is understood that the various components,reactants, solvents, temperatures, and conditions of the invention canalso be used in connection with the combined injection method to producethe nanoparticles of the invention.

In a further aspect, a temperature sufficient to initiate nanoparticleformation and sufficient to support nanoparticle growth can be fromabout 250° C. to about 400° C., for example, from about 275° C. to about350° C., from about 300° C. to about 350° C., from about 325° C. toabout 350° C., from about 300° C. to about 375° C., or from about 300°C. to about 400° C. In a further aspect, a temperature insufficient toinitiate nanoparticle formation but sufficient to support nanoparticlegrowth can be from about 150° C. to about 350° C., for example, fromabout 200° C. to about 300° C., from about 225° C. to about 300° C.,from about 250° C. to about 300° C., from about 275° C. to about 300°C., or from about 275° C. to about 325° C. In a further aspect, atemperature insufficient to initiate nanoparticle formation andinsufficient to support nanoparticle growth can be from about 100° C. toabout 300° C., for example, from about 150° C. to about 250° C., fromabout 175° C. to about 250° C., from about 200° C. to about 250° C.,from about 225° C. to about 250° C., from about 200° C. to about 225°C., from about 175° C. to about 200° C., from about 150° C. to about200° C., less than about 200° C., less than about 175° C., or less thanabout 150° C.

In one aspect, the heating step can comprise the steps of heating areaction mixture to a temperature sufficient to initiate nanoparticleformation and sufficient to support nanoparticle growth, and adding tothe reaction mixture an injection mixture comprising a source of cadmiumor zinc and a source of selenium, sulfur, or tellurium. In a furtheraspect, the heating step can comprise the steps of heating a reactionmixture comprising a source of cadmium or zinc to a temperaturesufficient to initiate nanoparticle formation and sufficient to supportnanoparticle growth, and adding to the reaction mixture an injectionmixture comprising a source of selenium, sulfur, or tellurium.

D. Theoretical

Typically, the ultra-small nanocrystals of the invention do not exhibitthe strong band edge emission feature that is observed in conventionalCdSe nanocrystals, but do exhibit strong band edge absorption featuresindicative of high quality CdSe nanocrystals. Without wishing to bebound by theory, it is believed that the broad emission can beattributed to charge recombination from surface midgap states that arisefrom the presence of non-coordinated surface selenium sites. See, e.g.,Hill, N. A.; Whaley, K. B. J. Chem. Phys. 1994, 100, 2831-2837. Whileband edge emission can occur by direct recombination of the electron andhole within a nanocrystal, deep trap emission can occur when aphotogenerated hole, trapped in a midgap state, encounters an electronbefore it can relax non-radiatively to the ground state. See, e.g.,Underwood, D. F.; Kippeny, T. C.; Rosenthal, S. J., Journal of PhysicalChemistry B 2001, 105, 436-443. That is, without wishing to be bound bytheory, it is believed that the observed properties are the directresult of the extreme surface-to-volume ratio forcing the electron andhole to predominately interact at the nanocrystal surface.

This phenomenon of hole trapping to the selenium surface sites has beenstudied by ultrafast fluorescence upconversion spectroscopy. Underwood,D. F.; Kippeny, T. C.; Rosenthal, S. J., Journal of Physical Chemistry B2001, 105, 436-443. Typically, as nanocrystal size decreases, the amountof hole trapping increases. Without wishing to be bound by theory, it isbelieved that this is due not only to the reduced physical distance thehole must travel to reach the surface, but also to an increasedsurface-to-volume ratio resulting in more available surface sites.Accordingly, as nanocrystal size continues to decrease, an even largerpopulation of photoexcited excitons can be funneled toward thehole-trapping decay pathway, ultimately making it the dominant mode ofradiative relaxation. Ultra-small nanocrystals are so small that theelectron wave function can have significant overlap with the seleniumsurface sites. Brus, L. E. J. Chem. Phys 1984, 80; Brus, L. J. Phys.Chem. 1986, 90, 2555-2560. Therefore, any hole trapped on the surfacewould likely encounter the electron before non-radiatively relaxing tothe ground state. Underwood, D. F.; Kippeny, T. C.; Rosenthal, S. J.Journal of Physical Chemistry B 2001, 105, 436-443. Compounding thissituation, in the present invention, nanocrystal growth time is so short(typically, from about 10 seconds to about 20 seconds) that surfacereconstruction and high temperature annealing have little time to occur.Without wishing to be bound by theory, it is believed that this resultsin a surface that is likely defect-ridden. Furthermore, unlike largernanocrystals, the diameter of the ultra-small nanocrystal and the lengthof the ligand are quite comparable. Again, without wishing to be boundby theory, it is believed that coupling of vibrational modes of theligand to surface atoms, as well as collisional relaxation, can providefurther avenues for energy dissipation, thereby providing more effectivetrap sites and potentially contributing to the broad lognormal emissionline shape.

While deep trap emission can be quite common in small (<30 Å)nanocrystals, it is typically accompanied by a large band edge emissionfeature. Landes, C. F.; Braun, M.; El-Sayed, M. A. J. Phys. Chem. B2001, 105, 10554-10558; Murray, C. B.; Norris, D. J.; Bawendi, M. G. J.Am. Chem. Soc. 1993, 115, 8706-8715. In contrast to that observed in thepresent invention, the presence of a strong band edge feature can biasthe white-light emission toward a particular color, thereby reducing thequality of the white light produced. The band edge emission feature ofthe nanoparticles of the invention is greatly diminished, providing amore balanced white-light emission with chromaticity coordinates of0.322, 0.365, which fall well within the white region of the 1931 CIEdiagram (FIG. 2). See alsohttp://hyperphysics.phy-astr.gsu.edu/hbase/vision/cie.html#c2. Examplesof the white-light emission are shown in FIG. 3. FIG. 4 compares theemission spectrum of ultra-small CdSe nanocrystals to conventionaltungsten (General Electric 100 W) and fluorescent lighting (SylvaniaOctron® #F032) emission spectra with the AM 1.5 solar spectrum, and acommercial white LED (Photon Micro-Light®, Photon Light.com). Thevisible spectrum is denoted by the shaded area.

E. Surface-Modified Ultra-Small Nanocrystals With Enhanced Quantum Yield

The synthesis, and subsequent functionality, of semiconductornanocrystals for a variety of applications has become a topic of broadscientific interest. [Kuno, M.; Lee, J. K.; Dabbousi, B. O.; Mikulec, F.V.; Bawendi, M. G., The band edge luminescence of surface modified CdSenanocrystallites: Probing the luminescing state. Journal of ChemicalPhysics 1997, 106, (23), 9869-9882.; Murray, C. B.; Norris, D. J.;Bawendi, M. G., Synthesis and Characterization of Nearly MonodisperseCde (E=S, Se, Te) Semiconductor Nanocrystallites. Journal of theAmerican Chemical Society 1993, 115, (19), 8706-8715.; Peng, Z. A.;Peng, X. G., Formation of high-quality CdTe, CdSe, and CdS nanocrystalsusing CdO as precursor. Journal of the American Chemical Society 2001,123, (1), 183-184.; Yin, Y.; Alivisatos, A. P., Colloidal nanocrystalsynthesis and the organic-inorganic interface. Nature 2005, 437, (7059),664-670.; Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.;Kadavanich, A.; Alivisatos, A. P., Shape control of CdSe nanocrystals.Nature 2000, 404, (6773), 59-61.; Rosenthal, S. J.; McBride, J.;Pennycook, S. J.; Feldman, L. C., Synthesis, surface studies,composition and structural characterization of CdSe, core/shell andbiologically active nanocrystals. Surface Science Reports 2007, 62,111-157.] In particular, colloidal nanocrystals have been investigatedfor use as light harvesters in photovoltaic devices, [Greenham, N. C.;Shinar, J.; Partee, J.; Lane, P. A.; Amir, O.; Lu, F.; Friend, R. H.,Optically detected magnetic resonance study of efficient two-layerconjugated polymer light-emitting diodes. Physical Review B 1996, 53,(20), 13528-13533.; Erwin, M. M.; Kadavanich, A. V.; McBride, J.;Kippeny, T.; Pennycook, S.; Rosenthal, S. J., Material characterizationof a nanocrystal based photovoltaic device. European Physical Journal D2001, 16, (1-3), 275-277.; Smith, N. J.; Emmett, K. J.; Rosenthal, S.J., Photovoltaic cells fabricated by electrophoretic deposition of CdSenanocrystals. Applied Physics Letters 2008, 93, (4).; Nozik, A. J.,Multiple exciton generation in semiconductor quantum dots. ChemicalPhysics Letters 2008, 457, (1-3), 3-11.; Klimov, V. I., Spectral anddynamical properties of multilexcitons in semiconductor nanocrystals.Annual Review of Physical Chemistry 2007, 58, 635-673.; Schaller, R. D.;Klimov, V. I., High efficiency carrier multiplication in PbSenanocrystals: Implications for solar energy conversion. Physical ReviewLetters 2004, 92, (18).; Ellingson, R. J.; Beard, M. C.; Johnson, J. C.;Yu, P. R.; Micic, O. I.; Nozik, A. J.; Shabaev, A.; Efros, A. L., Highlyefficient multiple exciton generation in colloidal PbSe and PbS quantumdots. Nano Letters 2005, 5, (5), 865-871.] as biological labels,[Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P.,Semiconductor nanocrystals as fluorescent biological labels. Science1998, 281, (5385), 2013-2016.; Rosenthal, S. J.; et al., Targeting cellsurface receptors with ligand-conjugated nanocrystals. Journal of theAmerican Chemical Society 2002, 124, (17), 4586-4594.; Warnement, M. R.;Tomlinson, I. D.; Rosenthal, S. J., Fluorescent imaging applications ofQuantum Dot probes. Current Nanoscience 2007, 3, (4), 273-284.; Chan, W.C. W.; Nie, S. M., Quantum dot bioconjugates for ultrasensitivenonisotopic detection. Science 1998, 281, (5385), 2016-2018.; Jaiswal,J. K.; Mattoussi, H.; Mauro, J. M.; Simon, S. M., Long-term multiplecolor imaging of live cells using quantum dot bioconjugates. NatureBiotechnology 2003, 21, (1), 47-51.; Kim, S.; et al. Near-infraredfluorescent type II quantum dots for sentinel lymph node mapping. NatureBiotechnology 2004, 22, (1), 93-97.] and as the emissive material inlight-emitting diodes (LED). [Colvin, V. L.; Schlamp, M. C.; Alivisatos,A. P., Light-Emitting-Diodes Made from Cadmium Selenide Nanocrystals anda Semiconducting Polymer. Nature 1994, 370, (6488), 354-357.; Achermann,M.; Petruska, M. A.; Kos, S.; Smith, D. L.; Koleske, D. D.; Klimov, V.I., Energy-transfer pumping of semiconductor nanocrystals using anepitaxial quantum well. Nature 2004, 429, (6992), 642-646.; Firth, A.V.; Cole-Hamilton, D. J.; Allen, J. W., Optical properties of CdSenanocrystals in a polymer matrix. Applied Physics Letters 1999, 75,(20), 3120-3122.; Schreuder, M. A.; Gosnell, J. D.; Smith, N. J.;Warnement, M. R.; Weiss, S. M.; Rosenthal, S. J., Encapsulatedwhite-light CdSe nanocrystals as nanophosphors for solid-state lighting.Journal of Materials Chemistry 2008, 18, (9), 970-975.; Anikeeva, P. O.;Madigan, C. F.; Halpert, J. E.; Bawendi, M. G.; Bulovic, V., Electronicand excitonic processes in light-emitting devices based on organicmaterials and colloidal quantum dots. Physical Review B 2008, 78, (8).;Coe, S.; Woo, W. K.; Bawendi, M.; Bulovic, V., Electroluminescence fromsingle monolayers of nanocrystals in molecular organic devices. Nature2002, 420, (6917), 800-803.]

Initial investigations of nanocrystals focused on the crystal structureand size as the sole source of the unique optoelectronic propertiesobserved. However, it has been determined that the surface ligands onthe nanocrystal are not only useful for their tethering and solubilityproperties, but are also a factor in the photoluminescent (PL)properties. [Kuno, M.; Lee, J. K.; Dabbousi, B. O.; Mikulec, F. V.;Bawendi, M. G., The band edge luminescence of surface modified CdSenanocrystallites: Probing the luminescing state. Journal of ChemicalPhysics 1997, 106, (23), 9869-9882.; Murray, C. B.; Norris, D. J.;Bawendi, M. G., Synthesis and Characterization of Nearly MonodisperseCde (E=S, Se, Te) Semiconductor Nanocrystallites. Journal of theAmerican Chemical Society 1993, 115, (19), 8706-8715.; Yin, Y.;Alivisatos, A. P., Colloidal nanocrystal synthesis and theorganic-inorganic interface. Nature 2005, 437, (7059), 664-670.;Pradhan, N.; Reifsnyder, D.; Xie, R. G.; Aldana, J.; Peng, X. G.,Surface ligand dynamics in growth of nanocrystals. Journal of theAmerican Chemical Society 2007, 129, (30), 9500-9509.; Bullen, C.;Mulvaney, P., The effects of chemisorption on the luminescence of CdSequantum dots. Langmuir 2006, 22, (7), 3007-3013.; Landes, C.; Braun, M.;Burda, C.; El-Sayed, M. A., Observation of large changes in the band gapabsorption energy of small CdSe nanoparticles induced by the adsorptionof a strong hole acceptor. Nano Letters 2001, 1, (11), 667-670.; Munro,A. M.; Ginger, D. S., Photoluminescence quenching of single CdSenanocrystals by ligand adsorption. Nano Letters 2008, 8, (8),2585-2590.; Munro, A. M.; Plante, I. J. L.; Ng, M. S.; Ginger, D. S.,Quantitative study of the effects of surface ligand concentration onCdSe nanocrystal photoluminescence. Journal of Physical Chemistry C2007, 111, (17), 6220-6227.; Liu, I. S. et al., Enhancingphotoluminescence quenching and photoelectric properties of CdSe quantumdots with hole accepting ligands. Journal of Materials Chemistry 2008,18, (6), 675-682.; Munro, A. M.; Bardecker, J. A.; Liu, M. S.; Cheng, Y.J.; Niu, Y. H.; Plante, I. J. L.; Jen, A. K. Y.; Ginger, D. S.,Colloidal CdSe quantum dot electroluminescence: ligands andlight-emitting diodes. Microchimica Acta 2008, 160, (3), 345-350.; Ji,X. H.; Copenhaver, D.; Sichmeller, C.; Peng, X. G., Ligand bonding anddynamics on colloidal nanocrystals at room temperature: The case ofalkylamines on CdSe nanocrystals. Journal of the American ChemicalSociety 2008, 130, (17), 5726-5735.; Kim, S. M.; Kyhm, K.; Yang, H. S.;Hong, K. S., Optical properties and surface conditions of CdSe quantumdots. Journal of the Korean Physical Society 2006, 49, S688-S691.;Smith, A. M.; Duan, H. W.; Rhyner, M. N.; Ruan, G.; Nie, S. M., Asystematic examination of surface coatings on the optical and chemicalproperties of semiconductor quantum dots. Physical Chemistry ChemicalPhysics 2006, 8, (33), 3895-3903.; Ozel, T. et al., Selectiveenhancement of surface-state emission and simultaneous quenching ofinterband transition in white-luminophor CdS nanocrystals usinglocalized plasmon coupling. New Journal of Physics 2008, 10, (8),083035.; Chen, X. B.; Samia, A. C. S.; Lou, Y. B.; Burda, C.,Investigation of the crystallization process in 2 nm CdSe quantum dots.Journal of the American Chemical Society 2005, 127, 4372-4375.; Wang,W.; Banerjee, S.; Jia, S. G.; Steigerwald, M. L.; Herman, I. P., Ligandcontrol of growth, morphology, and capping structure of colloidal CdSenanorods. Chemistry of Materials 2007, 19, (10), 2573-2580.; Washington,A. L.; Strouse, G. F., Microwave synthesis of CdSe and CdTe nanocrystalsin nonabsorbing alkanes. Journal of the American Chemical Society 2008,130, (28), 8916-8922.; Park, J.; Joo, J.; Kwon, S. G.; Jang, Y.; Hyeon,T., Synthesis of monodisperse spherical nanocrystals. AngewandteChemie-International Edition 2007, 46, 4630-4660.

Initially, conventional methodology used dimethylcadmium as the cadmiumprecursor in CdSe nanocrystal synthesis. [Murray, C. B.; Norris, D. J.;Bawendi, M. G., Synthesis and Characterization of Nearly MonodisperseCde (E=S, Se, Te) Semiconductor Nanocrystallites. Journal of theAmerican Chemical Society 1993, 115, (19), 8706-8715. ]However, due tothe cost and hazards associated with the use of dimethylcadmium, otherprecursors have been developed. [Peng, Z. A.; Peng, X. G., Formation ofhigh-quality CdTe, CdSe, and CdS nanocrystals using CdO as precursor.Journal of the American Chemical Society 2001, 123, (1), 183-184.;Rosenthal, S. J.; McBride, J.; Pennycook, S. J.; Feldman, L. C.,Synthesis, surface studies, composition and structural characterizationof CdSe, core/shell and biologically active nanocrystals. SurfaceScience Reports 2007, 62, 111-157.; Hambrock, J.; Birkner, A.; Fischer,R. A., Synthesis of CdSe nanoparticles using various organometalliccadmium precursors. Journal of Materials Chemistry 2001, 11, (12),3197-3201.; Qu, L. H.; Peng, Z. A.; Peng, X. G., Alternative routestoward high quality CdSe nanocrystals. Nano Letters 2001, 1, (6),333-337.] Specifically, CdO combined with a phosphonic acid to form aCd-phosphonate precursor complex was suggested as a more cost effectiveand green alternative. [Peng, Z. A.; Peng, X. G., Formation ofhigh-quality CdTe, CdSe, and CdS nanocrystals using CdO as precursor.Journal of the American Chemical Society 2001, 123, (1), 183-184.] As aresult of this synthetic innovation, the effects of the phosphonic acidas a component of the Cd precursor and as a surface ligand have beenexplored. [Nair, P. S.; Fritz, K. P.; Scholes, G. D., A multipleinjection method for exerting kinetic control in the synthesis of CdSenanorods. Chemical Communications 2004, 2084-2085.; Liu, H. T.; Owen, J.S.; Alivisatos, A. P., Mechanistic study of precursor evolution incolloidal group II-VI semiconductor nanocrystal synthesis. Journal ofthe American Chemical Society 2007, 129, (2), 305-312.; Kanaras, A. G.;Sonnichsen, C.; Liu, H. T.; Alivisatos, A. P., Controlled synthesis ofhyperbranched inorganic nanocrystals with rich three-dimensionalstructures. Nano Letters 2005, 5, (11), 2164-2167.] Phosphonic acidligands were shown to affect the growth kinetics and shape of thenanocrystals. [Wang, W.; Banerjee, S.; Jia, S. G.; Steigerwald, M. L.;Herman, I. P., Ligand control of growth, morphology, and cappingstructure of colloidal CdSe nanorods. Chemistry of Materials 2007, 19,(10), 2573-2580.; Yong, K. T.; Sahoo, Y.; Swihart, M. T.; Prasad, P. N.,Growth of CdSe quantum rods and multipods seeded by noble-metalnanoparticles. Advanced Materials 2006, 18, (15), 1978-+.; Nair, P. S.;Fritz, K. P.; Scholes, G. D., Evolutionary shape control duringcolloidal quantum-dot growth. Small 2007, 3, (3), 481-487.]

Ab-initio and first principles calculations have since determined thatphosphonic acid is the most strongly bound ligand on nanocrystals,leading to the surface coating being composed almost exclusively ofthem. [Manna, L. et al., First-principles modeling of unpassivated andsurfactant-passivated bulk facets of wurtzite CdSe: A model system forstudying the anisotropic growth of CdSe nanocrystals. Journal ofPhysical Chemistry B 2005, 109, (13), 6183-6192.; Peng, Z. A.; Peng, X.G., Nearly monodisperse and shape-controlled CdSe nanocrystals viaalternative routes: Nucleation and growth. Journal of the AmericanChemical Society 2002, 124, (13), 3343-3353.; Puzder, A.; Williamson, A.J.; Zaitseva, N.; Galli, G.; Manna, L.; Alivisatos, A. P., The effect oforganic ligand binding on the growth of CdSe nanoparticles probed by Abinitio calculations. Nano Letters 2004, 4, (12), 2361-2365.; Kopping, J.T.; Patten, T. E., Journal of the American Chemical Society 2008, 130,(17), 5689-5698.]

Without wishing to be bound by theory, for CdSe nanocrystals, theconventional paradigm has been that as the nanocrystal diameterdecreased, the band-edge absorption and band-edge emission shifted tohigher energies, due to quantum confinement. [Rossetti, R.; Ellison, J.L.; Gibson, J. M.; Brus, L. E., Size Effects in the Excited ElectronicStates of Small Colloidal Cds Crystallites. Journal of Chemical Physics1984, 80, (9), 4464-4469.] In contrast, for white-light emitting CdSenanocrystals, as shown in FIG. 12 (ultra-small sizes, <1.7 nm), theemission is no longer dominated by the band-edge emission. [Bowers, M.J.; McBride, J. R.; Rosenthal, S. J., White-light emission fromultra-small cadmium selenide nanocrystals. Journal of the AmericanChemical Society 2005, 127, (44), 15378-15379.] It has been shown thatalthough the absorption continues to blue-shift, indicating smallerdiameters, the bluest emission feature appears to be pinned at aspecific energy/wavelength (FIG. 12). [Dukes, A. D.; Schreuder, M. A.;Sammons, J. A.; McBride, J. R.; Smith, N. J.; Rosenthal, S. J., Pinnedemission from ultra-small cadmium selenide nanocrystals. Journal ofChemical Physics 2008, 129, (12).] This pinned emission is believed tobe due to trap states on the surface of the nanocrystal, influenced bythe surface ligands. [Bowers, M. J.; McBride, J. R.; Rosenthal, S. J.,White-light emission from ultra-small cadmium selenide nanocrystals.Journal of the American Chemical Society 2005, 127, (44), 15378-15379.;Dukes, A. D.; Schreuder, M. A.; Sammons, J. A.; McBride, J. R.; Smith,N. J.; Rosenthal, S. J., Pinned emission from ultra-small cadmiumselenide nanocrystals. Journal of Chemical Physics 2008, 129, (12).]

Previous reports have differed over whether the emission of theseultra-small nanocrystals can be altered through ligand modification.[Nag, A.; Sarma, D. D., White light from Mn²⁺-doped Cds nanocrystals: Anew approach. Journal of Physical Chemistry C 2007, 111, 13641-13644.;Singh, S. et al., Structure, microstructure and physical properties ofZnO based materials in various forms: bulk, thin film and nano. Journalof Physics D-Applied Physics 2007, 40, 6312-6327.] These discussionshave generally centered on a difference of opinion as to the source ofthis broad emission. Defect emission coming from either interstitialatoms or vacancies in the core of the nanocrystals would not be alteredby varying the surface ligands. CdSe nanocrystals with these types ofdefects would typically be more amorphous than crystalline due to thesmall number of inner atoms. Chen et al. have previously studied theabsorption and emission of nanocrystals of amorphous character. [Chen,X. B.; Samia, A. C. S.; Lou, Y. B.; Burda, C., Investigation of thecrystallization process in 2 nm CdSe quantum dots. Journal of theAmerican Chemical Society 2005, 127, 4372-4375.] This work showed thatamorphous nanocrystal defect emission should be accompanied byabsorption of mid-gap states, as well as a blue-shift in the absorptionpeak maxima due to crystallization over time. [Chen, X. B.; Samia, A. C.S.; Lou, Y. B.; Burda, C., Investigation of the crystallization processin 2 nm CdSe quantum dots. Journal of the American Chemical Society2005, 127, 4372-4375.] However, it has been shown that with thedisclosed methods, these ultra-small nanocrystal's absorption maximared-shifts due to growth of the crystal and not preliminarycrystallization. [Dukes, A. D.; Schreuder, M. A.; Sammons, J. A.;McBride, J. R.; Smith, N. J.; Rosenthal, S. J., Pinned emission fromultra-small cadmium selenide nanocrystals. Journal of Chemical Physics2008, 129, (12).] This work, along with the large Stokes shift seen inthis ultra-small size regime, has led to the discovery that thebroad-emission comes from surface states, indicating that this emissioncould be tuned by changing the ligand environment around those states.

In the disclosed methods, the effects that eleven different phosphonicacid ligands (FIG. 13) have on the photoluminescence properties ofwhite-light emitting nanocrystals are evaluated. Changing the phosphonicacid ligand on the surface of these ultra-small nanocrystals allowstuning of the highest energy peak in the emission and affects thequantum yield. In addition to the fundamental interest in controllingtrap-state emission, tuning this emission will allow for color controlover white solid-state light sources that employ nanocrystals as theemissive source.

F. Frequency Converters

In one aspect, the invention relates to a frequency converter. That is,at least one quantum dot of the invention, nanoparticle of any of theinvention, plurality of nanoparticles of the invention, or product ofthe methods of the invention, or mixture thereof, can be dispersedwithin a matrix, for example a polymeric or glass matrix, and placedwithin the emission path of an energy source providing light above theband gap energy of the nanoparticle of the invention. Thus, thenanoparticles of the invention can act as frequency downconverters whenexcited by light above the band gap energy (see FIG. 1). Accordingly,the nanoparticles can emit white-light when encapsulated in a polymermatrix and excited by, for example, an ultraviolet (UV) emittinglight-emitting diode (LED) (see FIG. 3). Different polymers (e.g.,polyurathane, polycarbonate, polymethylmethacralate, polyesters) as wellas varied nanocrystal loading, can be employed to optimize theperformance of a frequency converter device and, therefore, obtain peakefficiencies. In one aspect, a commercially available polyurethane, suchas that found in MINWAX®, can be used to provide the polymeric matrix.

In one aspect, a frequency converter can be constructed by encapsulatingat least one nanoparticle of the invention within a polymer matrix(e.g., polyurethane) and positioning the resulting composition withinthe emission path of an energy source providing light above the band gapenergy of the nanoparticle of the invention.

In one aspect, the polymer matrix can comprise any polymer known tothose of skill in the art. For example, the polymer matrix can compriseat least one of a polyurethane, a polyacrylate, a polymethacrylate, apolycarbonate, a polyester, a polyamide, a polyimide, a polyether, apolyolefin, a polystyrene, a polythiophene, a polysiloxane, or apolyfluoroethylene, or a mixture thereof, or a copolymer thereof. In oneaspect, the polymer is substantially transparent to visible light. In afurther aspect, the polymer is substantially transparent to ultravioletlight. In one aspect, the polymer matrix comprises a photocurablepolymer. In a further aspect, a condensation polymer is substantiallyabsent from the polymer matrix. It is also understood that the polymercan include additives to modify the physical or chemical properties ofthe polymer. In one aspect, the polymer matrix can be formed by anymethod known to those of skill in the art, for example, by solutioncasting or by injection molding. Further, the polymer matrix can beprovided in any desired shape, for example, the polymer matrix can beprovided as a film or a lens. Alternatively, the polymer matrix can beformed by performing a polymerization reaction in the presence of atleast one nanoparticle of the invention.

In one aspect, the matrix can comprise any glass known to those of skillin the art. For example, the glass matrix can comprise borosilicateglass. In a further aspect, the glass matrix can further comprise atleast one additive known to those of skill in the art to modify thephysical or optical properties of the glass matrix. In a further aspect,an additive can be selected and included in the glass of the glassmatrix such that the additive improves the strength of the glass matrixor such that the additive absorbs particular wavelengths of lightemitted by the energy source or the nanoparticles of the invention. Forexample, the matrix—polymeric or glass—can be supplemented with anadditive that is opaque with respect to ultraviolet energy, yettransparent to visible light.

In a further aspect, the frequency converter of the invention canfurther comprise at least one phosphor, additional quantum dot ormixture thereof. In such aspects, the at least one phosphor, additionalquantum dot, or mixture thereof can be selected such that the at leastone phosphor, additional quantum dot, or mixture thereof can absorbenergy from the energy source providing light above the band gap energyof the nanoparticle of the invention or from the emission of thenanoparticle of the invention and can emit energy of the same or of adifferent frequency as the nanoparticle of the invention, therebymodifying the emission of the frequency converter. For example, thephosphor can comprise at least one white phosphor or yellow phosphor ora mixture thereof. In such an aspect, the phosphor can absorb energyfrom the energy source (e.g., UV light) and emit visible light of afrequency intrinsic to the at least one white phosphor or yellowphosphor or a mixture thereof.

In a further aspect, the additional quantum dot can be a quantum dot ofother than the nanoparticle of the invention. In such an aspect, theadditional quantum dot can absorb energy from the energy sourceproviding light above the band gap energy of the nanoparticle of theinvention or from the emission of the nanoparticle of the invention andcan emit energy of the same or of a different frequency as thenanoparticle of the invention, thereby modifying the emission of thefrequency converter.

In a further aspect, the additional quantum dot can function as anenergy cascade system of the invention, as disclosed herein, and iscapable of absorbing energy of a wavelength outside the firstelectromagnetic region and capable of emitting energy in the firstelectromagnetic region. That is, in such an aspect, the additionalquantum dot can absorb energy emitted by the energy source providinglight above the band gap energy of the nanoparticle of the invention andcan emit energy within the absorption region of the nanoparticle of theinvention, thereby increasing the amount of energy for absorption by thenanoparticle of the invention and, therefore, increasing the energyemitted by the frequency converter. For example, the additional quantumdot can be capable of absorbing energy of a wavelength of from about 100nm to about 290 nm and capable of emitting energy of a wavelength offrom about 290 nm to about 400 nm. As another example, the additionalquantum dot can be capable of absorbing energy of a wavelength of fromabout 100 nm to about 320 nm and capable of emitting energy of awavelength of from about 320 nm to about 400 nm. In one aspect, theadditional quantum dot comprises at least one of cadmium sulfide,cadmium selenide, cadmium telluride, zinc selenide, zinc sulfide, orzinc telluride or a mixture thereof. For example, the quantum dot cancomprise a single material (e.g., cadmium sulfide), a core/shell or twoor more materials (e.g., cadmium sulfide/zinc sulfide), or an alloy oftwo or more materials.

G. Light Emitting Diode Devices

In one aspect, a light emitting diode device can be provided by usingthe frequency converter of the invention with an energy source, forexample a light emitting diode, capable of providing energy above theband gap energy of the nanoparticle of the invention. That is, thefrequency converter can be positioned within the emission path of such,thereby providing a light emitting diode device.

Therefore, in one aspect, the invention relates to a light emittingdiode device comprising a light emitting diode (LED) capable of emittingenergy of a first wavelength, and the frequency converter of theinvention positioned within an emission path of the light emittingdiode, wherein the frequency converter is capable of absorbing energy ofthe first wavelength. In a further aspect, the first wavelengthcomprises energy within the first electromagnetic region of thenanoparticle of the invention employed in the frequency converter of theinvention. For example, the first wavelength can comprise light having awavelength of from about 200 nm to about 500 nm, of from about 300 nm toabout 450 nm, of from about 100 nm to about 290 nm, of from about 290 nmto about 400 nm, of from about 100 nm to about 320 nm, or of from about320 nm to about 400 nm.

In a further aspect, the light emitting diode device of the inventioncan further comprise at least one phosphor, additional quantum dot ormixture thereof. In such aspects, the at least one phosphor, additionalquantum dot, or mixture thereof can be selected such that the at leastone phosphor, additional quantum dot, or mixture thereof can absorbenergy from the energy source providing light above the band gap energyof the nanoparticle of the invention or from the emission of thenanoparticle of the invention and can emit energy of the same or of adifferent frequency as the nanoparticle of the invention, therebymodifying the emission of the light emitting diode device. For example,the phosphor can comprise at least one white phosphor or yellow phosphoror a mixture thereof. In such an aspect, the phosphor can absorb energyfrom the energy source (e.g., UV light) and emit visible light of afrequency intrinsic to the at least one white phosphor or yellowphosphor or a mixture thereof.

In a further aspect, the additional quantum dot can be a quantum dot ofother than the nanoparticle of the invention. In such an aspect, theadditional quantum dot can absorb energy from the energy sourceproviding light above the band gap energy of the nanoparticle of theinvention or from the emission of the nanoparticle of the invention andcan emit energy of the same or of a different frequency as thenanoparticle of the invention, thereby modifying the emission of thelight emitting diode device.

In a further aspect, the additional quantum dot can function as anenergy cascade system of the invention, as disclosed herein, and iscapable of absorbing energy of a wavelength outside the firstelectromagnetic region and capable of emitting energy in the firstelectromagnetic region. That is, in such an aspect, the additionalquantum dot can absorb energy emitted by the energy source providinglight above the band gap energy of the nanoparticle of the invention andcan emit energy within the absorption region of the nanoparticle of theinvention, thereby increasing the amount of energy for absorption by thenanoparticle of the invention and, therefore, increasing the energyemitted by the light emitting diode device. For example, the additionalquantum dot can be capable of absorbing energy of a wavelength of fromabout 100 nm to about 290 nm and capable of emitting energy of awavelength of from about 290 nm to about 400 nm. As another example, theadditional quantum dot can be capable of absorbing energy of awavelength of from about 100 nm to about 320 nm and capable of emittingenergy of a wavelength of from about 320 nm to about 400 nm. In oneaspect, the additional quantum dot comprises at least one of cadmiumsulfide, cadmium selenide, cadmium telluride, zinc selenide, zincsulfide, or zinc telluride or a mixture thereof. For example, thequantum dot can comprise a single material (e.g., cadmium sulfide), acore/shell or two or more materials (e.g., cadmium sulfide/zincsulfide), or an alloy of two or more materials.

In one aspect, the light emitting diode device can be provided by amethod comprising the step of positioning the frequency converter of theinvention within an emission path of a light emitting diode (LED)capable of emitting energy of a first wavelength, wherein the frequencyconverter is capable of absorbing energy of the first wavelength. In afurther aspect, the positioning step can comprises the steps ofdissolving at least one polymer and dispersing at least one quantum dotof the invention, nanoparticle of any the invention, plurality ofnanoparticles of the invention, or product of the invention, or mixturethereof into a solvent, thereby producing a solution-dispersion;applying the solution-dispersion within the emission path of the lightemitting diode; and removing the solvent. That is, in one aspect, alight emitting diode device of the invention can be provided bysolution-casting a solution-dispersion of the nanoparticle of theinvention and a polymer onto at least a portion of the surface of andwithin the emission path of a light emitting diode (LED).

In a further aspect, the positioning step can comprise the steps ofdissolving at least one polymer and dispersing at least one quantum dotof the invention, nanoparticle of the invention, plurality ofnanoparticles of any of the invention, or product of the methods of theinvention, or mixture thereof into a solvent, wherein the polymer is aphotocurable polymer at the first wavelength, thereby producing aphotocurable solution-dispersion; contacting the emission path of thelight emitting diode with the photocurable solution-dispersion; andenergizing the light emitting diode, thereby photocuring at least aportion of the photocurable solution-dispersion. That is, in one aspect,a light emitting diode device of the invention can be provided byphotocuring (also referred to as photocrosslinking) asolution-dispersion of the nanoparticle of the invention and a polymeronto at least a portion of the surface of and within the emission pathof a light emitting diode (LED). Photocurable polymers are well-known tothose of skill in the art, and it is understood that any photocurablepolymer can be used in connection with the light emitting diode deviceof the invention. For example, the photocurable polymer can comprisepolybutadiene or a copolymer of polybutadiene. In one aspect, theresulting photocured polymer is substantially transparent to visiblelight. In a further aspect, the resulting photocured polymer issubstantially transparent to ultraviolet light.

In a further aspect, the positioning step can comprise the steps ofdissolving at least one monomer and at least one photoactive initiatorand dispersing at least one quantum dot of the invention, nanoparticleof the invention, plurality of nanoparticles of the invention, orproduct of the methods of the invention, or mixture thereof into asolvent, wherein the photoactive initiator is active at the firstwavelength and the photoactive initiator can initiate polymerization ofthe monomer, thereby producing a photopolymerizable solution-dispersion;contacting the emission path of the light emitting diode with thephotopolymerizable solution-dispersion; and energizing the lightemitting diode, thereby photopolymerizing at least a portion of themonomer dissolved in the photopolymerizable solution-dispersion. Thatis, in one aspect, a light emitting diode device of the invention can beprovided by photopolymerizing a solution-dispersion of the nanoparticleof the invention and a polymer onto at least a portion of the surface ofand within the emission path of a light emitting diode (LED).Photoactive initiators and photopolymerizable polymers are well-known tothose of skill in the art, and it is understood that any photoactiveinitiator and/or photopolymerizable polymer can be used in connectionwith the light emitting diode device of the invention. For example, thephotopolymerizable polymer can comprise polymethyl methacrylateinitiated by 2,2-dimethoxy-2-phenylacetophenone and1-hydroxycyclohexylphenyl ketone. In one aspect, the resultingphotopolymerizable polymer is substantially transparent to visiblelight. In a further aspect, the resulting photopolymerizable polymer issubstantially transparent to ultraviolet light.

H. Modified Fluorescent Light Sources

In one aspect, the invention relates to a modified fluorescent lightsource. That is, a modified fluorescent light source can be provided bysubstituting the nanoparticle of the invention for the phosphor employedwithin a conventional fluorescent light source. In such an aspect, amodified fluorescent light source can comprise a substantially opticallytransparent and substantially hermetically sealed tube having a firstend, a second end, an interior surface, an exterior surface, and a lumenextending therethrough; a first electrode positioned at the first end; asecond electrode positioned at the second end; inert gas vapor andmercury vapor within the lumen of the tube; a phosphor substitutecomprising at least one frequency converter of the invention, quantumdot of the invention, nanoparticle of the invention, plurality ofnanoparticles of any of the invention, or product of the methods of theinvention, or mixture thereof substantially coating the interior surfaceor the exterior surface of the tube.

In one aspect, the modified fluorescent light source operates in amanner similar to the operation of a conventional fluorescent lightsource. For example, by establishing a current between the firstelectrode positioned at the first end and the second electrodepositioned at the second end, a plasma is established from the inert gasvapor and mercury vapor within the lumen of the substantially opticallytransparent and substantially hermetically sealed tube. In one aspect,the inert gas can comprise any inert gas capable of establishing aplasma under electrical current sustainable under conventionalelectrical loads; for example, the inert gas can comprise argon gas. Aswell-known to those of skill in the art, a plasma comprising mercuryemits ultraviolet light at a wavelength of 254 nm. In a conventionalfluorescent light source, the emitted ultraviolet light excites thephosphor coating at the interior surface of the tube, and the phosphorabsorbs the ultraviolet light and then emits visible light at discretewavelengths (see FIG. 4). It is also understood that other ultravioletsources—for example, Xenon or one or more halogens—can be used inconnection with the invention.

In contrast, in one aspect, in the modified fluorescent light source ofthe invention, the phosphor is replaced or supplemented by thenanoparticle of the invention. That is, the phosphor substitute can bepresent at the surface of the tube in combination with the phosphor orthe phosphor substitute can replace the phosphor. In one aspect, thephosphor is absent from the modified fluorescent light source. In oneaspect, the emitted ultraviolet light excites the nanoparticle of theinvention coating at the surface(s) of the tube, and the nanoparticleabsorbs the ultraviolet light and then emits broad emission visiblelight (see FIG. 4). In a further aspect, in the modified fluorescentlight source of the invention, the optically transparent andsubstantially hermetically sealed tube can comprise an ultraviolet lightopaque material and the phosphor substitute can substantially coat theinterior surface of the tube. In a further aspect, the opticallytransparent and substantially hermetically sealed tube can comprise anultraviolet light transparent material and the phosphor substitute cansubstantially coat the exterior surface of the tube.

In a further aspect, the modified fluorescent light source of theinvention can further comprise at least one phosphor, additional quantumdot or mixture thereof. In such aspects, the at least one phosphor,additional quantum dot, or mixture thereof can be selected such that theat least one phosphor, additional quantum dot, or mixture thereof canabsorb energy from the mercury emission or from the emission of thenanoparticle of the invention and can emit energy of the same or of adifferent frequency as the nanoparticle of the invention, therebymodifying the emission of the modified fluorescent light source. Forexample, the phosphor can comprise at least one white phosphor or yellowphosphor or a mixture thereof. In such an aspect, the phosphor canabsorb energy from the mercury emission (e.g., UV light) and emitvisible light of a frequency intrinsic to the at least one whitephosphor or yellow phosphor or a mixture thereof.

In a further aspect, the additional quantum dot can be a quantum dot ofother than the nanoparticle of the invention. In such an aspect, theadditional quantum dot can absorb energy from the mercury emission orfrom the emission of the nanoparticle of the invention and can emitenergy of the same or of a different frequency as the nanoparticle ofthe invention, thereby modifying the emission of the modifiedfluorescent light source.

In a further aspect, the additional quantum dot can function as anenergy cascade system of the invention, as disclosed herein, and iscapable of absorbing energy of a wavelength outside the firstelectromagnetic region and capable of emitting energy in the firstelectromagnetic region. That is, in such an aspect, the additionalquantum dot can absorb energy emitted by the mercury emission and canemit energy within the absorption region of the nanoparticle of theinvention, thereby increasing the amount of energy for absorption by thenanoparticle of the invention and, therefore, increasing the energyemitted by the modified fluorescent light source. For example, theadditional quantum dot can be capable of absorbing energy of awavelength of from about 100 nm to about 290 nm and capable of emittingenergy of a wavelength of from about 290 nm to about 400 nm. As anotherexample, the additional quantum dot can be capable of absorbing energyof a wavelength of from about 100 nm to about 320 nm and capable ofemitting energy of a wavelength of from about 320 nm to about 400 nm. Inone aspect, the additional quantum dot comprises at least one of cadmiumsulfide, cadmium selenide, cadmium telluride, zinc selenide, zincsulfide, or zinc telluride or a mixture thereof. For example, thequantum dot can comprise a single material (e.g., cadmium sulfide), acore/shell or two or more materials (e.g., cadmium sulfide/zincsulfide), or an alloy of two or more materials.

I. Electroluminescent Devices Based on Broad-Emission Nanocrystals

The unique optical properties of the nanoparticles of the invention makethem ideal materials for incorporation into solid state lightingapplications in the form of light emitting diodes (LEDs). LEDs can bemade by incorporating an intrinsic emitting layer into a p-n junctiondevice. When under forward bias, electrons and holes are injected intothe intrinsic layer, where they recombine to emit light. Lumileds™:Light From Silicon Valley, Craford, M. G. (2004). The wavelength of theemitted photon is determined by the intrinsic layer or phosphor.Typically, phosphors are limited to monochromatic emission, requiring amixture of phosphors or a complex doping scheme to achieve white-light.Yang, W.-J.; Luo, L.; Chen, T.-M.; Wang, N.-S. Chem. Mater. 2005, 17,3883-3888.

An ideal material for a white-light emitting intrinsic layer would berobust, emit over the entire visible spectrum, have high quantumefficiency, not suffer from self absorption, be easy to produce on alarge scale, and not waste energy producing wavelengths beyond thevisible spectrum (e.g., tungsten). Taking these criteria intoconsideration, semiconductor nanocrystals, in particular thenanoparticles of the invention, are attractive candidate materials forapplications in solid state lighting. Unlike commercial phosphors,emission color can be tuned by simply controlling the nanocrystal size.Further, white-light emission can be provided by employing thenanoparticles of the invention.

Nanocrystal-based LEDs have been demonstrated showing colored-lightemission. One example developed by Klimov et al. utilizes core/shellnanocrystals deposited onto an InGaN quantum well. The quantum well ispumped by laser emission, generating electrons and holes which, throughenergy transfer, cause the nanocrystals to emit light at theirfundamental band edge energy. Achermann, M.; Petruska, M. A.; Kos, S.;Smith, D. L.; Koleske, D. D.; Klimov, V. I. Nature 2004, 429, 642-646.While intuitive, simply mixing several colors of nanocrystals togetherto achieve white light results in an overall reduction of deviceefficiency through self-absorption between the various sizes ofnanocrystals. This can be especially detrimental for device designsrequiring a thickness of more than several monolayers. Mueller, A. H.;Petruska, M. A.; Achermann, M.; Werder, D. J.; Akhadov, E. A.; Koleske,D. D.; Hoffbauer, M. A.; Klimov, V. I. Nano Lett. 2005, 5, 1039-1044.The large Stokes shift and narrow size distribution of these ultra-smallnanocrystals, along with the broad emission spectrum, would alleviateany efficiency loss attributed to self-absorption from mixingnanocrystal sizes.

In contrast to conventional mixed-nanocrystal-based electroluminescentdevices, the nanoparticle of the invention, in one aspect, when used asan intrinsic layer, can provide a broadband emission, thereby providingwhite light while avoiding an overall reduction of device efficiencythrough self-absorption between the various sizes of nanocrystals

1. Structure

In one aspect, the electroluminescent device comprises an n-typesemiconductor, a p-type semiconductor, and a quantum dot layer inelectrical or photonic communication with the n-type semiconductor andthe p-type semiconductor, wherein the quantum dot layer comprises atleast one quantum dot of the invention, nanoparticle of the invention,plurality of nanoparticles of any of the invention, product of themethods of the invention, or frequency converter of the invention, ormixture thereof (see, e.g., an exemplary electroluminescent devicestructure in FIG. 5). It is understood that n-type semiconductor andp-type semiconductor materials—both inorganic and organic (e.g.,N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine(TPD), aluminum tris(8-hydroxyquinoline) (Alq₃), or other knownpolymeric semiconductors)—known to those of skill in the art can be usedin connection with the nanoparticles of the invention. In a furtheraspect, the quantum dot layer can be in contact with and in electricalcommunication with the n-type semiconductor and the p-typesemiconductor. That is, in one aspect, the nanoparticle of the inventioncan be excited through contact-based energy transfer interactions. In afurther aspect, the quantum dot layer can be in contact with the n-typesemiconductor or the p-type semiconductor and in electrical or photoniccommunication with the n-type semiconductor and the p-typesemiconductor. That is, in one aspect, the nanoparticle of the inventioncan be excited through non-contact-based energy transfer interactions.

In a further aspect, the electroluminescent device of the invention canfurther comprise at least one phosphor, additional quantum dot ormixture thereof. In such aspects, the at least one phosphor, additionalquantum dot, or mixture thereof can be selected such that the at leastone phosphor, additional quantum dot, or mixture thereof can absorbenergy from the light emitted by recombination of electrons and holesinjected into the intrinsic layer under forward bias or from theemission of the nanoparticle of the invention and can emit energy of thesame or of a different frequency as the nanoparticle of the invention,thereby modifying the emission of the electroluminescent device. Forexample, the phosphor can comprise at least one white phosphor or yellowphosphor or a mixture thereof. In such an aspect, the phosphor canabsorb energy from the light emitted by recombination of electrons andholes injected into the intrinsic layer under forward bias (e.g., UVlight) and emit visible light of a frequency intrinsic to the at leastone white phosphor or yellow phosphor or a mixture thereof.

In a further aspect, the additional quantum dot can be a quantum dot ofother than the nanoparticle of the invention. In such an aspect, theadditional quantum dot can absorb energy from the light emitted byrecombination of electrons and holes injected into the intrinsic layerunder forward bias or from the emission of the nanoparticle of theinvention and can emit energy of the same or of a different frequency asthe nanoparticle of the invention, thereby modifying the emission of theelectroluminescent device.

In a further aspect, the additional quantum dot can function as anenergy cascade system of the invention, as disclosed herein, and iscapable of absorbing energy of a wavelength outside the firstelectromagnetic region and capable of emitting energy in the firstelectromagnetic region. That is, in such an aspect, the additionalquantum dot can absorb energy emitted by the light emitted byrecombination of electrons and holes injected into the intrinsic layerunder forward bias and can emit energy within the absorption region ofthe nanoparticle of the invention, thereby increasing the amount ofenergy for absorption by the nanoparticle of the invention and,therefore, increasing the energy emitted by the electroluminescentdevice. For example, the additional quantum dot can be capable ofabsorbing energy of a wavelength of from about 100 nm to about 290 nmand capable of emitting energy of a wavelength of from about 290 nm toabout 400 nm. As another example, the additional quantum dot can becapable of absorbing energy of a wavelength of from about 100 nm toabout 320 nm and capable of emitting energy of a wavelength of fromabout 320 nm to about 400 nm. In one aspect, the additional quantum dotcomprises at least one of cadmium sulfide, cadmium selenide, cadmiumtelluride, zinc selenide, zinc sulfide, or zinc telluride or a mixturethereof. For example, the quantum dot can comprise a single material(e.g., cadmium sulfide), a core/shell or two or more materials (e.g.,cadmium sulfide/zinc sulfide), or an alloy of two or more materials.

2. Device Fabrication

Electroluminescent devices of the invention can be prepared by methodsanalogous to those reported in the work of Coe et al., who employ arelatively inorganic approach to nanocrystal-based electroluminescentdevices (FIG. 5). Coe, S.; Woo, W.; Bawendi, M.; Bulovic, V.;“Electroluminescence from single monolayers of nanocrystals in molecularorganic devices,” Nature 420 800-803 (2002). Briefly, a chloroformsolution of ultra-small CdSe andN,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′diamine(TPD), can be spin-cast onto a clean indium tin oxide (ITO) coated glasssubstrate. The nanocrystal and TPD concentrations can be adjusted toyield a monolayer of nanocrystals, on top of an about 35 nm TDP layer,when spin-cast. An approximately 40 nm thick film of aluminumtris(8-hydroxyquinoline) (Alq₃) can then be thermally evaporated;followed by an approximately 1 mm diameter, approximately 75 nm thickMg:Ag (10:1 by mass) cathode with a 50 nm Ag cap. An additional layer of3-(4-biphenyl)-4-phenyl-5-t-butylphenyl-1,2,4-triazole (TAZ) and/orbathocuprione (BCP) with a nominal thickness of 10 nm can be addedbetween the nanocrystals and the Alq₃ layer or between the Alq₃ layerand the metal cathode layers. All of these processes are typicallycarried out in a dry box under air-free nitrogen atmosphere. Moreadvanced designs can incorporate Bragg mirrors or other photonic crystalstructures, which can enhance the brightness, efficiency, anddirectionality of light emission. In these devices, a bottom mirror canessentially reflect back any light emitted toward the substrate andtherefore increase the light extraction through top surface.

3. Device Characterization

The properties that can be evaluated for the broad-emissionnanocrystal-based LEDs in solid state lighting applications are luminousintensity, luminous efficacy, color temperature, and lifetime. In orderto measure these properties, photometric experiments that measureoptical radiation as perceived by the human eye can be performed. Aphotometric quantity can be obtained as follows: P=∫K(λ)R(λ)dλ, where Prepresents the photometric quantity (e.g., luminous flux in lumens), Rrepresents the corresponding radiometric quantity that can be measureddirectly (e.g., radiant flux in Watts), and K(λ) represents the visualresponse of the eye as a function of wavelength. Therefore, photometricexperiments can use either a broadband detector with a special photopicfilter that is calibrated to mimic the spectral response of the humaneye (i.e., photometer) or a photodiode array with associated computersoftware that takes into account the spectral luminous efficacy of thehuman eye (i.e., spectroradiometer). Spectroradiometers are moreaccurate, especially at the edges of the visual response curve.

Traditional light source testing methods are typically insufficient forcharacterizing LEDs due to the complicated spatial light distributionthat is typical for LEDs. Special consideration is taken to avoid largediscrepancies in measurements from different metrology laboratories.Therefore, in addition to maintaining a constant temperature environmentand drive current, specific geometrical measurement conditions areimposed to ensure reproducibility. The International Commission onIllumination (CIE) sets the standards for characterizing an LED'sperformance. FIG. 6 illustrates one such configuration. The LED can beplaced in a fixed position with its mechanical center aligned to themiddle of the detector, which is located at a distance, D, from the LED.The optical center, meaning the location of the center of the emissioncone of light, is not necessarily coincident with the mechanical center.Consequently, in order to enable reproducible measurements, all LEDs aretested at their mechanical center. The CIE has suggested the A and Bcriterion for setting the distance, D. Measurements are taken at D equalto 316 mm (A) and 100 mm (B). While the two conditions should give thesame results for a Lambertian source that emits light uniformly in alldirections, the emission of LEDs tends to be non-uniform and directionalso the two measurements need to be averaged to determine thestandardized LED performance. The configuration shown in FIG. 6 can beused to measure average luminous intensity (lm/sr), which looselycorrelates to brightness as perceived by the human eye.

A second configuration is necessary to measure the total luminous flux(lm), lifetime, and color. By placing the LED in the center of anintegrating sphere, the flux emitted in all directions can be detected.Given the electrical power input to the LED and the measured luminousflux output, the efficiency of the device can be determined. Theindustry target for luminous efficacy, as specified in the OIDASolid-State LED Roadmap, is 75 lm/W by 2007 and 200 lm. Light EmittingDiodes (LEDs) for General Illumination: An OIDA Technology RoadmapUpdate 2002(http://lighting.sandia.gov/lightingdocs/OIDA_SSL_LED_Roadmap_Full.pdf).For comparison, an incandescent lamp has an efficiency of approximately16 lm/W, a commercial white light LED operates at approximately 30 lm/W,and a research-grade white light LED has been demonstrated atapproximately 80 lm/W for very low drive currents. Seehttp://www.lumileds.com; N. Narendran, Y. Gu, J. P. Freyssinier-Nova,and Y. Zhu, “Extracting phosphor-scattered photons to improve white LEDefficiency,” Phys. Stat. Sol. A 202, R60-R62 (2005). Internal quantumefficiency, light extraction efficiency, and phosphor down-conversionefficiency are all contributing factors to luminous efficacy. Thelifetime of a solid-state LED, considered to be the 50% lumendepreciation level, can also be determined using the integrating sphere.The target lifetime for solid-state LEDs is greater than 100,000 hoursby 2020. Current white light LEDs have lifetimes greater than 50,000hours. http://www.lumileds.com. If the integrating sphere is coupledwith a spectroradiometer, a calculation of the chromaticity coordinatesand color temperature can also be accomplished, which specifies theapparent color of the LED when viewed directly. The spectroradiometerperforms a complete spectral power distribution of the source beingmeasured from which the colorimetric parameters can be mathematicallycalculated.

J. Energy Cascade Systems

The nanoparticle of the invention typically absorbs energy within afirst electromagnetic region and emits energy within a secondelectromagnetic region. When used in connection with an energy source,the first electromagnetic region of the nanoparticle of the inventioncan, in one aspect, can comprise wavelengths of energy other than theentire emission region of the energy source. That is, the energy sourcemay emit light at one or more wavelengths outside of the absorbance bandof the nanoparticle of the invention. In such aspects, an energy cascadesystem can be employed to accumulate energy for absorption by thenanoparticle. The resulting system can be termed a cascade of energycollected for use by the longer-wavelength-absorbing nanoparticle orquantum dot. Specifically, in one aspect, a nanocrystal or core/shellnanocrystal, including for example a nanoparticle of the invention, canbe used to collect light at very low wavelengths, e.g., 250 nm. Thecore/shell can then emit light at a higher wavelength (e.g., within afirst electromagnetic region), wherein a nanoparticle in the inventioncan absorb efficiently.

For example, in one aspect, when an energy source emits energy within afirst absorption electromagnetic region and the nanoparticle or quantumdot absorbs energy within a second absorption electromagnetic region,wherein the first absorption electromagnetic region and the secondabsorption electromagnetic region do not wholly overlap, thenanoparticle or quantum dot cannot absorb all available energy emittedby the energy source. In such an aspect, at least two quantum dots inphotonic or energetic communication, wherein the first quantum dot iscapable of absorbing energy from a first absorption electromagneticregion and capable of emitting energy in a first emissionelectromagnetic region, wherein the second quantum dot is capable ofabsorbing energy from a second absorption electromagnetic region andcapable of emitting energy in a second emission electromagnetic region,and wherein the first emission electromagnetic region overlaps with thesecond absorption electromagnetic region can be employed to make agreater amount of energy emitted by the energy source available forabsorption by the nanoparticle or quantum dot.

The energy cascade systems of the invention can be used in connectionwith the nanoparticle of the invention as well as with nanoparticles ofother than the invention. That is, in one aspect, the energy cascadesystem comprises a second quantum dot comprising the quantum dot of theinvention, the nanoparticle of the invention, or the product of themethods of the invention. In a further aspect, both the first quantumdot and the second quantum dot are narrow band emission nanocrystals ofother than the invention.

In one aspect, the invention relates to an energy cascade systemcomprising at least two quantum dots in photonic communication, whereinthe first quantum dot is capable of absorbing energy from a firstabsorption electromagnetic region and capable of emitting energy in afirst emission electromagnetic region, wherein the second quantum dot iscapable of absorbing energy from a second absorption electromagneticregion and capable of emitting energy in a second emissionelectromagnetic region, and wherein the first emission electromagneticregion overlaps with the second absorption electromagnetic region.

In a further aspect, the first absorption electromagnetic regioncomprises wavelengths of from about 100 nm to about 290 nm and the firstemission electromagnetic region comprises wavelengths of from about 290nm to about 400 nm. In a yet further aspect, the first absorptionelectromagnetic region comprises wavelengths of from about 100 nm toabout 320 nm and the first emission electromagnetic region compriseswavelengths of from about 320 nm to about 400 nm. In a still furtheraspect, the second absorption electromagnetic region compriseswavelengths of from about 290 nm to about 400 nm or from about 320 nm toabout 400 nm. In one aspect, the second emission electromagnetic regioncomprises wavelengths of from about 400 nm to about 700 nm or from about420 nm to about 710 nm.

In one aspect, the first quantum dot comprises at least one of cadmiumsulfide, cadmium selenide, cadmium telluride, zinc selenide, zincsulfide, or zinc telluride or a mixture thereof. For example, thequantum dot can comprise a single material (e.g., cadmium sulfide), acore/shell or two or more materials (e.g., cadmium sulfide/zincsulfide), or an alloy of two or more materials.

In one aspect, the second quantum dot comprises the nanoparticle of theinvention or at least one of cadmium sulfide, cadmium selenide, cadmiumtelluride, zinc selenide, zinc sulfide, or zinc telluride or a mixturethereof. For example, the quantum dot can comprise a single material(e.g., cadmium sulfide), a core/shell or two or more materials (e.g.,cadmium sulfide/zinc sulfide), or an alloy of two or more materials.

K. Solid State Lighting

Solid state lighting, in the form of white light emitting diodes (LEDs),is an attractive replacement for current lighting technologies based ontheir potential for longer service lifetimes and lower powerconsumption. [The Promise of Solid State Lighting for GeneralIllumination, Optoelectronics Industry Development Association,Washington, D.C., 2001.] This requires an LED that emits white lightwhich is pleasing to the eye, similar to the output from incandescentlight bulbs or sunlight. Most current white-light LEDs emit a harshwhite color similar to fluorescent lights. The quality of the emittedwhite light can be evaluated by quantities such as CIE chromaticitycoordinates, color rendering index (CRI), and correlated colortemperature (CCT). Each of these measures is important forcharacterizing the unique properties of white-light LEDs. The CIEchromaticity coordinates (x, y) allow specification of the exact colorof the emission as perceived by the human eye, where (0.33, 0.33) is theideal value for white light. Color rendering index is a measure of theability of a source to reproduce the colors of a variety of objects thatit illuminates, presented on a scale of 0 to 100, representing the worst(0) and best (100) color rendering. Correlated color temperature is ameasure of the temperature in degrees Kelvin where a blackbody radiatorbest matches the emission spectrum of the tested light source. Ideal CCTvalues for white-light LEDs range from warmer white (2500 K to 4500 K)to cooler white (4500 K to 6500 K). [D'Andrade, B. W.; Forrest, S.R.“White Organic Light-Emitting Devices for Solid-State Lighting.” Adv.Mater. 16, 1585-1595, 2004.] The values for each of these quantities aregiven for common white-light sources in Table 1.

A 2001 Department of Energy study found that transitioning to solidstate lighting, such as using white-light LEDs, would reduce carbondioxide emission by 258 million metric tons and eliminate the need forover 100 new power stations over a 20 year period. [The Promise of SolidState Lighting for General Illumination, Optoelectronics IndustryDevelopment Association, Washington, D.C., 2001.] Generally, the goal isto decrease the global electricity usage for lighting by 50% while atthe same time provide higher quality lighting. Over the same 20 yearperiod, this could save 760 GW of energy in the U.S. alone, which wouldresult in a financial savings of over $115 billion dollars.

TABLE 1 CIE coordinates, CRI, and CCT for common white-light sources.White-Light Source CIE x CIE y CRI CCT (K) Incandescent Bulb 0.448 0.408100 2854 Fluorescent, cool white 0.375 0.367 89 4080 Fluorescent, warmwhite 0.440 0.403 72 2940 Daylight (CIE Standard Illuminant D₆₅) 0.3130.329 90 6500 Inova commercial white-light LED 0.308 0.300 76 7137[D'Andrade, B. W.; Forrest, S. R. “White Organic Light-Emitting Devicesfor Solid-State Lighting.” Adv. Mater. 16, 1585-1595, 2004.] The Inovacommercial white-light LED was measured in a Labsphere SLMS-LED-1050integrating sphere system.

1. White-Light LEDs

A current problem with transitioning to solid state lighting is the lackof an efficient LED that is easy to manufacture and possesses sufficientcolor properties for general lighting. Several approaches have beenreported toward the development of a viable solid state white-light LED.One technique utilizes a blue light-emitting InGaN LED coated withcerium-doped yttrium aluminum garnet (Ce³⁺:YAG) crystals, which functionas a yellow phosphor. [Tamura, T.; Setomoto, T.; Taguchi, T.“Illumination characteristics of lighting array using 10 candela-classwhite LEDs under AC 100 V operation.” Journal of Luminescence 87-89,1180-1182, 2000.; Schlotter, P., et al., “Fabrication andcharacterization of GaN:InGaN:AlGaN double heterostructure LEDs andtheir application in luminescence conversion LEDs.” Mat. Sci. and Eng. B59, 390-394, 1999.] The crystals convert some of the blue LED light toyellow light, creating a mixture of yellow and blue light that gives theappearance of white light. This light is considered to be cool whitebecause it has a color temperature of about 5000 K. The lack of emissionin the red region of the spectrum results in an inferior white lightcompared to incandescent bulbs, which is a significant obstacle toovercome before achieving the transition to solid state lighting. Inorder to enable a more tunable form of white light, a combination ofred, green, and blue phosphors can be pumped with a UV LED.[Steigerwald, D. A., et al., “Illumination With Solid State LightingTechnology.” IEEE J. on Sel. Top. in Quant. Elec. 8, 310-320, 2002.;Sheu, J. K., et al., “White-Light Emission From Near UV InGaN—GaN LEDChip Precoated With Blue/Green/Red Phosphors.” IEEE Photonics TechnologyLetters 15, 18-20, 2003.] However, by using three phosphors instead ofone, the complexity of fabricating these devices is greatly increased.

White-light LEDs have also been demonstrated without the use ofphosphors by growing homoepitaxial zinc selenide (ZnSe) on a ZnSesubstrate. Some of the greenish-blue light (485 nm) from the activeregion of the p-n junction is absorbed by the substrate and re-emittedas yellow light (centered on 585 nm) via self-activated emission,resulting from distant donor-acceptor pair transitions. [Katayama, K.,et al., “ZnSe-based white LEDs.” Journal of Crystal Growth 214/215,1064-1070, 2000.; Shirakawa, T. “Effect of defects on the degradation ofZnSe-based white LEDs.” Mater. Sci. and Eng. B 91-92, 470-475, 2002.;Thomas, A. E.; Russell, G. J.; Woods, J. “Self-activated emission in ZnSand ZnSe.” J. Phys. C: Solid State Phys. 17, 6219-6228, 1984.] However,the color rendering of these devices is poor due to a CRI of 68, whichis not high enough for general room illumination. White-light LEDs havealso been fabricated with higher color rendering by using separate red,green, and blue LEDs to produce white light with a CRI greater than 80.[Muthu, S.; Schuurmans, F. J.; Pashley, M. D. “Red, Green, and Blue LEDsfor White Light Illumination.” IEEE J. on Sel. Top. In Quant. Elec. 8,333-338, 2002.; Chhajed, S.; Xi, Y.; Li, Y.-L.; Gessmann, Th.; Schubert,E. F. “Influence of junction temperature on chromaticity andcolor-rendering properties of trichromatic white-light sources based onlight-emitting diodes.” J. of Appl. Phy. 97, 054506, 2005.] However,these devices are expensive and complex to produce because of thecomplicated feedback system that is necessary to maintain the propercolor balance as the red, green, and blue LEDs degrade at differentrates over time.

2. CdSe Nanocrystal-Based LEDs

For more than a decade, monochromatic light emitting diodes have beenfabricated using CdSe nanocrystals as the emitting layer of the device.Electrons are injected into the nanocrystal layer while holes areinjected into a layer of a semiconducting polymer such asp-paraphenylene vinylene (PPV) or polyvinylcarbazole (PVK), resulting inlight emission from both the polymer and the CdSe layer. [Colvin, V. L.;Schlamp, M. C.; Alivisatos, A. P. “Light-emitting diodes made fromcadmium selenide nanocrystals and a semiconducting polymer.” Nature 370,354-357, 1994.; Dabbousi, B. O.; Bawendi, M. G.; Onitsuka, O.; Rubner,M. F. “Electroluminescence from CdSe quantum-dot/polymer composites.”Appl. Phys. Lett. 66, 1316-1318, 1995.; Schlamp, M. C.; Peng, X.;Alivisatos, A. P. “Improved efficiencies in light emitting diodes madewith CdSe.CdS.core/shell type nanocrystals and a semiconductingpolymer.” J. of Appl. Phys. 82, 5837-5842, 1997.; Mattoussi, H., et al.,“Electroluminescence from heterostructures of poly(phenylene vinylene)and inorganic CdSe nanocrystals.” J. of Appl. Phys. 83, 7965-7974,1998.; Mattoussi, H., et al., “Composite thin films of CdSe nanocrystalsand a surface passivating/electron transporting block copolymer:Correlations between film microstructure by transmission electronmicroscopy and electroluminescence.” J. of Appl. Phys. 86, 4390-4399,1999.] Because of the poor charge conduction in CdSe nanocrystal layers,utilizing a single monolayer of CdSe nanocrystals has been shown toincrease the external electroluminescence quantum efficiency of thesedevices from <0.01% to 1.1%. [Coe, S.; Woo, W.-K.; Bawendi, M.; Bulović,V. L. “Electroluminescence from single monolayers of nanocrystals inmolecular organic devices.” Nature 420, 800-803, 2002.; Coe-Sullivan,S.; Woo, W.-K.; Steckel, J. S.; Bawendi, M. G.; Bulović, V. L. “Tuningthe performance of hybrid organic/inorganic quantum dot light-emittingdevices.” Org. Electron. 4, 123-130, 2003.] Recently, a new chargeconduction mechanism has been demonstrated where CdSe nanocrystals aredeposited onto an InGaN quantum well and electron-hole pairs areindirectly injected into the nanocrystals, resulting in an energytransfer and band edge emission by the nanocrystals. [Achermann, M.;Petruska, M. A.; Kos, S.; Smith, D. L.; Koleske, D. D.; Klimov, V. I.“Energy-transfer pumping of semiconductor nanocrystals using anepitaxial quantum well.” Nature 429, 642-646, 2004.] In addition, CdSenanocrystals have been used as the intrinsic layer of an all-inorganicLED when surrounded by n- and p-doped GaN to inject electrons and holesdirectly, avoiding the low carrier mobilities of organic materials.[Mueller, A. H. et al. “Multicolor Light-Emitting Diodes Based onSemiconductor Nanocrystals Encapsulated in GaN Charge Injection Layers.”Nano Lett. 5, 1039-1044, 2005.]

3. White-Light Emitting CdSe Nanocrystals

Current white-light, nanocrystal-based devices rely on either acombination of nanocrystal and polymer emission or the mixing of severalsizes of nanocrystals to achieve white light. It has been widelypublished that the color of the light emitted depends on the size of thenanocrystal. For example, CdSe nanocrystals with a diameter of 7 nm emitred light while CdSe nanocrystals with a diameter of 2 nm emit bluelight. [Murray, C. B., Norris, D. J., Bawendi, M. G. “Synthesis andCharacterization of Nearly Monodisperse CdE (E=S, Se, Te) SemiconductorNanocrystallites.” J. Am. Chem. Soc. 115, 8706-8715, 1993.; Alivisatos,A. P. “Semiconductor Clusters, Nanocrystals, and Quantum Dots.” Science271, 933-937, 1996.] The efficiency of white-light LEDs based on severalsizes of nanocrystals is compromised due to the overlap of the emissionand absorption spectra of the different size nanocrystals. The presentapproach to achieving high quality white-light LEDs that can befabricated at low cost involves the use of white-light emitting CdSenanocrystals that are a single ultra-small size and exhibit broadbandemission from 420-710 nm when illuminated with UV light, as seen inFIG. 1. These nanocrystals are monodisperse with a diameter ofapproximately 1.5 nm. Without wishing to be bound by theory, because theultra-small nanocrystals are believed to be approximately 1.5 nm indiameter, the emission in the blue part of the spectrum is believed dueto the inherent size of the nanocrystals and is likely a result ofinternal exciton recombination processes. The broad emission in theyellow and red portions of the ultra-small CdSe nanocrystal emissionspectrum has been attributed to midgap states resulting fromuncoordinated selenium atoms on the surface of the nanocrystals.[Bowers, M. J., II; McBride, J. R.; Rosenthal, S. J. “White-LightEmission from Ultra-small Cadmium Selenide Nanocrystals.” J. Am. Chem.Soc. 127, 15378-15379, 2005.]

In one aspect, the invention relates to a broad emission LED comprisingat least one nanocrystal of the invention encapsulated within a polymer,for example, a polymer as disclosed herein, including but not limited toa perfluorocyclobutyl (PFCB) polymer. In various aspects, LEDscomprising the nanocrystals of the invention can emit produce whitelight from 420-710 nm.

4. Advantages of White-Light Nanocrystal-Based Devices

Single, ultra-small CdSe nanocrystals, in one aspect, have a largeStokes shift separating the absorption and emission peaks. Consequently,devices based on these nanocrystals will not suffer fromself-absorption, potentially resulting in higher device efficiency.Another advantage is that ultra-small CdSe nanocrystal white-light LEDswould not require a mixture of phosphors or a complex feedback system togenerate white light, resulting in much simpler devices. Furthermore,the addition of emission in the red portion of the spectrum gives highercolor rendering than in devices emitting mostly blue and yellow light,such as the blue LED with yellow phosphor and the ZnSe-based devices.Encapsulating ultra-small CdSe nanocrystals coated onto UV LEDs act asfrequency downconverters to produce white light from 420-710 nm.

In the preparation of LEDs, the choice of encapsulant can play a role inthe white-light emission spectrum. In the examples, the highest qualitywhite-light spectra were observed when the ultra-small CdSe nanocrystalsare dispersed into PFCB polymer. Increasing the concentration ofnanocrystals in PFCB typically results in the emission color changingfrom a cooler white to a warmer white. Furthermore, varying thewavelength of the LED used to excite the samples can have a small effecton the emission spectrum of the nanocrystals, with longer wavelengthsLEDs causing a shift towards a cooler white color. Using a 385 nm LED toexcite 2% CdSe nanocrystals in PFCB, chromaticity coordinates of (0.33,0.30), CRI of 79, a color temperature of 5406 K, and luminous efficacyslightly below 0.5 lm/W have been measured. Incorporation of ultra-smallnanocrystals into an electroluminescent device can lead to higherquality and more economical solid state lighting devices.

L. EXPERIMENTAL

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how thecompounds, compositions, articles, devices and/or methods claimed hereinare made and evaluated, and are intended to be purely exemplary of theinvention and are not intended to limit the scope of what the inventorsregard as their invention. Efforts have been made to ensure accuracywith respect to numbers (e.g., amounts, temperature, etc.), but someerrors and deviations should be accounted for. Unless indicatedotherwise, parts are parts by weight, temperature is in ° C. or is atambient temperature, and pressure is at or near atmospheric.

Acetonitrile, ethyl acetate, hexanes, toluene, methanol, hexanol, andbutanol, all reagent grade, were purchased from Sigma-Aldrich. For thenanocrystal synthesis, phenyl phosphonic acid (PPA, 98%, Aldrich), hexylphosphonic acid (97%, Strem), hexadecyl amine (HDA, technical grade 90%,Aldrich), CdO (99.99%, Strem), Se powder (200 mesh, 99.99%, Strem), andtributyl phosphine (TBP, 97%, Aldrich) were used as received, whileother phosphonic acids were synthesized as described below. The variousphosphonic acid syntheses required the use HCl (12.1 M, aqueous, EMDChemicals), triethyl phosphite (TEP, 98%, Aldrich), and eitherBromodocosane (96%, Aldrich), Bromohexadecane (97%, Aldrich),Bromododecane (97%, Aldrich), Bromodecane (98%, Aldrich), Bromooctane(99%, Acros-organics), Bromobutane (98%, Aldrich), 2-ethyl,hexyl bromide(95%, Aldrich), 3-methyl,butyl bromide (96%, Aldrich), or3-phenyl,propyl bromide (98%, Acros-organics).

All ¹H and ¹³C spectra were recorded on a Bruker 300 MHz spectrometerequipped with a 7 Tesla Oxford magnet controlled by a Bruker DPX-300console at ambient temperature in deuterated DMSO, methanol, or CDCl₃.The spectra were analyzed using TopSpin 2.0 software. The IR spectra forall the phosphonic acids were recorded on a ThermoNicolet—IR300spectrometer using EZOMNIC version 6.1 software. ESI-MS measurementswere done on a FinniganMat—Electrospray Ionization LCQ Mass spectrometersystem with TunePlus version 1.3 software. All absorption measurementswere conducted on a Varian-Cary 50 Bio UV-Visible Spectrophotometer withaccompanying software. The emission spectra were recorded using an ISSPC1 photon counting spectrofluorimeter, using Vinci version 1.6.SP5software.

1. Synthesis of Ultra-Small Cadmium Selenide Nanocrystals

a. Reagents

Tri-n-octylphosphine oxide (TOPO, 90% tech. grade), hexadecylamine (HDA,90% technical grade) and octadecene (ODE, 90% technical grade) werepurchased from Aldrich and used as received. CdO (99.999% Puratrem),tri-n-butylphosphine (TBP, 97%), and selenium powder (200 mesh) werepurchased from Strem and used as received. Dodecylphosphonic acid (DPA)was synthesized via the Arbuzov reaction from triethylphoshite and1-dodecylbromide followed by acid work up with concentrated HCl andrecrystallization from cold ethyl acetate. [Maege, I.; Jaehne, E.;Henke, A.; Adler, H.-J. P.; Bram, C.; Jung, C.; Stratmann, M.“Self-assembling adhesion promoters for corrosion resistant metalpolymer interfaces.” Progress in Organic Coatings 34, 1-12, 1998.] Allother solvents used were HPLC grade and purchased from FisherScientific, unless otherwise noted.

b. Procedure 1

A 1.0 M solution of selenium in TBP was produced by dissolving 7.896 gselenium powder in 100 mL of TBP. The 0.10 M injection mixture was madeby dilution of the 1.0 M Se:TBP solution with ODE. This solution wasstable for several days stored in a capped bottle, in ambient conditionsat room temperature. The reaction solvent was mixed from a unit quantityof TOPO and HDA (7.2 g and 2.97 g respectively) along with 0.128 g ofCdO and 0.496 g of DPA (scaled as necessary). These contents were heatedin a 100 mL three neck flask under argon with vigorous stirring to 310°C. A needle was placed in the septum to allow for an argon purge untilthe reaction mixture reached 150° C., at which point the reaction vesselwas considered water- and oxygen-free. Upon reaching reactiontemperature (330° C.), 5 mL of the Se:TBP:ODE solution were swiftlyinjected and the temperature reduced to 270° C. as quickly as waspossible without allowing the temperature to drop below 260° C. Toachieve ultra-small nanocrystals (e.g., <20 Å), a second syringe oftoluene (typically 5 mL) was injected to reduce the reaction temperatureto <150° C. within about 2 to about 10 seconds after the initialinjection.

c. Procedure 2

As before, a 0.10 M injection mixture of selenium in TBP was produced bydissolving 7.896 g selenium powder in 100 mL of TBP and dilution of the1.0 M Se:TBP solution with ODE. The reaction mixture was prepared from2.97 g of HDA and 0.128 g of CdO and 0.496 g of DPA (scaled asnecessary). These contents were heated in a 100 mL three neck flaskunder argon with vigorous stirring to 330° C., until the solution becamecolorless. The mixture was then purged with argon and cooled toapproximately room temperature (i.e., about 25° C.). Then, 7.2 g of TOPOwas added to the reaction mixture, which was then heated to reactiontemperature (e.g., about 330° C.).

At reaction temperature, 5 mL of the Se:TBP:ODE injection mixture wereinjected and the temperature reduced to 270° C. as quickly as waspossible without allowing the temperature to drop below 260° C. Toachieve ultra-small nanocrystals (e.g., <20 Å), a second syringe oftoluene (typically 5 mL) was injected to reduce the reaction temperatureto <150° C. within about 2 to about 10 seconds after the initialinjection.

d. Characterization

FIG. 1 shows the absorption and emission properties of a sample ofultra-small CdSe prepared by the methods of the invention. The firstabsorption feature is at 414 nm which has been assigned as athermodynamically determined “magic size,” on the order of 15 Å, forCdSe. Qu, L.; Yu, W. W.; Peng, X. Nano Lett. 2004, 4, 465-469; Landes,C.; Braun, M.; Burda, C.; El-Sayed, M. A. Nano Lett. 2001, 1, 667-670.The emission spectrum shows a band edge emission followed by twofeatures attributed to emission from energetically different midgapstates. The fluorescence quantum yield for this example is on the orderof 2% to 3%, with a calculated extinction coefficient of 22311 per moleof nanocrystals. Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003,15, 2854-2860. Additionally, the material demonstrated goodphotostability. Thin films of ultra-small CdSe (neat on a glass slide)were able to maintain their optical properties after ten days ofexposure to intense UV light from a 1000 μW, 370 nm LED (LEDtronics)under ambient conditions (air at room temperature).

2. Preparation of Nanocrystals by Etching (Comparative)

A solution of 456 nm absorbing CdSe nanocrystals in toluene was mixedwith butylamine. The final butylamine concentration was approximately0.9 M. The etching process was carried out at room temperature over fourdays. The absorption and emission spectra were collected once theprimary absorption peak had reached 414 nm. The spectrum of theresulting nanocrystals is shown in FIG. 7.

3. Ultra-Small Cadmium Selenide Nanocrystals Having Improved QuantumYield

a. Reagents

Tri-n-octylphosphine oxide (TOPO, 90% tech. grade), hexadecylamine (HDA90% tech. grade) and octadecene (ODE, 90% tech. grade) were purchasedfrom Aldrich and used as received. CdO (99.999% Puratrem),tri-n-butylphosphine (TBP, 97%), and selenium powder (200 mesh) werepurchased from Strem and used as received. Dodecylphosphonic acid (DPA)was synthesized via the Abruzov reaction from triethylphoshite and1-dodecylbromide followed by acid work up with concentrated HCl andrecrystallization from cold ethyl acetate. All other solvents were HPLCgrade and purchased from Fisher Scientific unless otherwise noted.

b. Synthesis

A 1.0 M solution of selenium in TBP was produced by dissolving 7.896 gselenium powder in 100 mL of TBP. The 0.20 M injection solution was madeby dilution of the 1.0 M Se:TBP solution with TBP. This solution wasstable for several days stored in a sealed bottle, in ambient conditionsat room temperature. The reaction solvent was mixed from a unit quantityof TOPO and HDA (7.0 g and 3.0 g respectively) along with 0.128 g of CdOand 0.496 g of DPA. These contents were heated in a 100 mL three neckflask under argon with vigorous stirring to 330° C. A needle was placedin the septum to allow for an argon purge until the reaction solutionreached 150° C. at which point the reaction vessel was considered waterand oxygen free. Upon reaching reaction temperature (330° C.), thereaction continued stirring until the red color from the CdOdisappeared. The temperature was then reduced to 315° C. and the heatingmantle was removed. 4.5 mL of the 0.2 M Se:TBP along with 2 mL ODE wereswiftly injected. A second syringe of butanol (typically 10 mL) wasinjected to reduce the reaction temperature to <150° C. within 2-10seconds after the initial injection. Time is determined by the observedcolor of the reaction vessel. That is, for example, the alcohol solutioncan be added at the first appearance of a yellow color in the solution.This addition, in one aspect, can also have the effect of precipitatingthe nanocrystals. This reaction may be scaled to produce larger orsmaller quantities.

c. Isolation

Nanocrystals are divided into equally into two 11 dram vials. The vialsare filled to the top with methanol and centrifuged. The liquid isdiscarded and a small amount of hexyl alcohol is added the vial and thencentrifuged again. The nanocrystals are removed by the hexyl alcoholleaving behind the excess HDA. The liquid is poured into two clean 11dram vials and the nanocrystals are precipitated by addition of methanoland collected by centrifugation. The solvent is discarded and thenanocrystals are re-dissolved in the non-polar solvent of choice(toluene, hexanes, chloroform, etc.).

d. Characterization

The nanocrystals of this example exhibit a broad emission spectrum. Anexemplary spectrum is shown as FIG. 8.

e. Quantum Yield

Nanocrystals produced in this example exhibit average quantum yields(i.e., light in to light out ratios) of at least 10%. That is, in thisexample, nanocrystals produced by the methods of the invention can havea quantum yield of greater than about 10%.

4. Encapsulation

a. Nanocrystal Synthesis

A 1.0 M stock solution of selenium in TBP was produced by dissolving11.84 g selenium powder in 150 mL of TBP. The injection solution wasproduced by further diluting a 0.2 M Se:TBP solution with TBP. Thissolution was stable for several days stored in a capped bottle, undernitrogen or argon at room temperature. The reaction solvent was mixedfrom a unit quantity of TOPO and HDA (7.0 g and 3.0 g respectively)along with 0.128 g of CdO and 0.496 g of DPA (scaled as necessary).These chemicals were placed in a 100 mL three neck flask, with atemperature probe, a bump trap, and a septum in the three necks. Aneedle was placed in the septum to allow for an argon purge until thereaction solution reached 150° C., at which point the reaction vesselwas considered water and oxygen free.

The reaction continued to be heated under argon with vigorous stirringup to 320° C. Upon reaching reaction temperature, 4.25 mL of the 0.2 MSe:TBP solution along with 2.00 mL of ODE solution were mixed in asyringe and swiftly injected. Following this injection the temperatureof the reaction flask was maintained between 270° C. and 240° C. Toachieve ultra-small nanocrystals (e.g., <20 Å), a second syringe ofbutanol (typically 10 mL) was injected to reduce the reactiontemperature to <130° C. within 2-10 seconds after the initial injection(depending on the desired size). Further cooling using compressed air,applied to the outside of the reaction flask, cooled the solution to<100° C. The nanocrystals were then removed from the coordinatingsolvents using methanol and hexanol washes, before being suspended intoluene.

b. Sample Preparation

A range of different encapsulant materials were chosen based on theirability to add phosphor materials to them before curing and on theircolor after curing, preferably clear. The encapsulants were preparedaccording to the instructions provided using the following materials:Easy Cast clear casting epoxy (Environmental Technology Inc.), RTVS 61clear silicone (Insulcast), Epo-Tek epoxy resin (Epoxy Technology),perfluorocyclobutyl (PFCB) polymer (Tetramer Technologies), EP 691 clearepoxy resin (Resinlab), and EP 965 clear epoxy resin (Resinlab). Othermaterials were tested but are not listed due to incomplete mixing withthe nanocrystal solution or insufficient hardening at room temperatureafter addition of the nanocrystals. The CdSe nanocrystals were addedwhile in solution (toluene) and mixed thoroughly before the encapsulantwas cured. Aggregation was monitored via fluorescence microscopy, wherethe emitted white light blinking on and off from a thin film ofnanocrystals in the encapsulant was observed. This on/off orintermittency behavior is based on an Auger ionization process and isonly seen in single nanocrystal emission, indicating little or noaggregation. [Nirmal, M.; Dabbousi, B. O.; Bawendi, M. G.; Macklin, J.J.; Trautman, J. K.; Harris, T. D.; Brus, L. E. “Fluorescenceintermittency in single cadmium selenide nanocrystals.” Nature 383,802-804, 1996.; Blanton, S. A.; Hines, M. A.; Guyot-Sionnest, P.“Photoluminescence wandering in single CdSe nanocrystals.” Appl. Phys.Lett. 69, 3905-3907, 1996.]

For curing, vials of samples were placed inside a lab oven at 65-100° C.for 3-24 hours depending on the curing schedule of the encapsulant.After hardening, the samples were removed from the oven and left to coolto room temperature before testing. If the encapsulant material did notadhere to the glass vial, the sample was removed from the vial beforetesting. However, if the encapsulant did adhere to the glass, the samplewas tested while still inside the vial. The Easy Cast, RTVS 61, Epo-Teksamples were tested after being removed from the vial, while the PFCB,EP 691, and EP 965 samples were tested in the vial.

c. Sample Testing

Absorption and photoluminescence were recorded for all samples. The UV-Vis absorbance measurements were performed in air using a Varian Cary Bio100 UV-Vis spectrophotometer. Photoluminescence measurements werecarried out using either a spectrofluoremeter or an integrating spherewith a fiber-coupled spectrometer. Multiple excitation sources were usedfor the photoluminescence measurements, including a 300 W high-pressurexenon arc lamp with monochromator, and UV or blue LEDs in wavelengths of365, 375, 385, 395, 405, 410, and 423 nm (Roithner LaserTechnik). TheLEDs were powered at 20 mA and 4 V using a Keithley 2400 sourcemeter.

The first photoluminescence apparatus utilized an ISS PC1 photoncounting spectrofluoremeter in air. The nanocrystals in solution wereexcited by the xenon white-light source using a monochromator. Thesamples of nanocrystals in encapsulant were excited using commercial UVand blue LEDs. Images of the LEDs and then the samples on top of theLEDs can be seen in FIG. 9, which shows the white light being emitted bythe samples. The absorption and emission spectra of these samples can beseen in FIG. 10 and FIG. 11, respectively. The second photoluminescenceapparatus consisted of a Labsphere SLMS-LED-1050 integrating spheresystem. The samples were placed on top of the LEDs in the center of theintegrating sphere. The integrating sphere was fiber-optically coupledto a CDS 500 CCD-based spectrometer that measured emission from 350-850nm. The provided software calculated radiant flux, luminous flux, CIEchromaticity coordinates, correlated color temperature (CCT), and colorrendering index (CRI).

Though many encapsulant materials were tested, the polymer PFCBtypically performed the best due to the high UV absorption by thesamples with CdSe nanocrystals at low thicknesses (several millimeters).To achieve this same level of absorption in other materials, most neededto be several centimeters thick, which is much larger than is typicallyacceptable for a commercial product. Ideally, either a very thin coatingof the encapsulant with nanocrystals would be applied to the outside ofa commercial UV LED, or the nanocrystals in the encapsulant wouldreplace the commercial clear epoxy bulb and encase a UV LED die. Ineither case, it is typically desirable to have as little as possible UVlight leak out of the device. In addition to the high absorbance, thenanocrystals mixed thoroughly and easily in PFCB while curing in amatter of hours. Based on its better performance than the otherencapsulants, the data will be presented only for PFCB samples.

d. Absorption

FIG. 10 shows the absorption spectra for PFCB alone, and PFCB with 1%,2%, 5%, and 10% nanocrystals by weight. The PFCB curve shows a gradualdecrease in absorption from 350 nm to 700 nm with no visible peaks. Thenanocrystals in solution show a strong band edge absorption feature at414 nm, which is typical for ultra-small CdSe nanocrystals with adiameter of 1.5 nm, as seen in FIG. 1. The nanocrystals with PFCB curvesshow the same general trend of the PFCB only curve with the addition ofthe band edge absorption feature at 414 nm, which shows that thenanocrystals have not grown in size and are dispersed into the polymermatrix. At only 1% nanocrystals by weight, the absorption spectrum isalmost identical to the polymer only absorption spectrum. At 2%nanocrystals in PFCB, the band edge absorption peak of the CdSenanocrystals can be observed. As the ratio of the nanocrystals to thepolymer increases, the absorption spectrum more strongly resembles thatof pure, ultra-small CdSe nanocrystals in solution.

e. Photoluminescence

FIG. 11 shows the emission spectra for PFCB alone and for PFCB with 1%,2%, 5%, and 10% nanocrystals by weight. A 365 nm LED was used as theexcitation source and the LED emission was detected with the photoncounting spectrofluoremeter. The PFCB curve shows a strong emission inthe blue portion of the spectrum up to about 500 nm, with very littleemission at higher wavelengths. At only 1% nanocrystals in PFCB, theemission spectrum shows a similar trend to the polymer only curve butwith a slightly lower amount of blue emission. The samples withconcentrations of 2%, 5%, and 10% nanocrystals display a decreasingamount of blue emission at higher concentrations of nanocrystals inPFCB. Like the absorbance, an increase in the amount of nanocrystals inthe sample causes the emission to appear more like the nanocrystal onlyemission spectrum and less like the PFCB only emission spectrum.

The color characteristics of CIE coordinates, CRI, and CCT as well asthe luminous efficacy for the PFCB with nanocrystal samples are reportedin Table 2. The luminous efficacy is calculated as total luminous fluxdivided by the electrical power used to light the UV LEDs, in lm/W. The1% and 2% concentration of nanocrystals in PFCB had CIE coordinates of(0.289, 0.260) and (0.328, 0.296) respectively when illuminated with 385nm LEDs, as seen in Table 2, which are within the white-light range andare similar to the other samples since the nanocrystals alone insolution have coordinates of (0.322, 0.365). [Bowers, M. J., II;McBride, J. R.; Rosenthal, S. J. “White-Light Emission from Magic-SizedCadmium Selenide Nanocrystals.” J. Am. Chem. Soc. 127, 15378-15379,2005.] The CRI and CCT were typically about 80 and 3500-5000 K,respectively, and are also in the ideal range for white-light LEDs.[Steigerwald, D. A., et al., “Illumination With Solid State LightingTechnology.” IEEE J. on Sel. Top. in Quant. Elec. 8, 310-320, 2002.] Theluminous efficacy of the exemplary devices is under 1 lm/W, which ispartly due to the low quantum yield (10%) of the white-lightnanocrystals. This yield is typically lower than the single-color CdSenanocrystal emission quantum yield.

TABLE 2 CIE coordinates, CRI, and CCT for 1%, 2%, 5%, and 10% by weightultra-small CdSe nanocrystals in PFCB when excited with three 365 nm,375 nm, or 385 nm LEDs. Luminous CCT Efficacy LED Concentration CIE xCIE y CRI (K) (lm/W) 365 1% 0.3566 0.4234 71.4 4779 0.05 nm 2% 0.37590.4143 78.1 4299 0.05 5% 0.4167 0.4896 66.7 3881 0.04 10%  0.4412 0.499360.4 3549 0.03 375 1% 0.3390 0.3817 81.2 5065 0.29 nm 2% 0.3855 0.415681.0 4029 0.29 5% 0.4217 0.4499 78.2 3559 0.27 10%  0.4377 0.4565 78.53346 0.25 385 1% 0.2886 0.2603 76.1 9497 0.49 nm 2% 0.3277 0.2955 79.05406 0.46 5% 0.3634 0.3407 80.5 4139 0.43 10%  0.3920 0.3675 81.1 35540.41

Results indicate that the excitation wavelength had a small effect onthe emission properties of the devices. As the peak wavelength of theLED increased, more blue light was emitted by the LED, resulting in acooler white color. This trend can be seen in Table 2, with higher colortemperatures resulting from excitation with longer wavelength LEDs. Incontrast, an increase in the concentration of nanocrystals in PFCBcaused a small decrease in the amount of blue emission from the device,partly due to self absorption of the nanocrystals. This resulted in awarmer white color or lower color temperature at higher concentrationsof nanocrystals in PFCB, also seen in Table 2. The luminous efficacy ofthe devices was similarly affected by these same factors. The level ofUV light absorbance by the samples varies with wavelength as shown inFIG. 10, which causes a change in the amount of emission power anddevice efficiency depending on the peak wavelength of the LED. In thesame way, the concentration of nanocrystals alters the efficiency of thesamples. By adding more nanocrystals to the same amount of polymer, moreUV light can be absorbed as well as emitted by the devices when theirconcentration is below 10%. In addition to the LED wavelength andconcentration, the emission of the devices can be affected by theencapsulant material used. Some of the materials used had a large amountof emission in the visible spectrum and can cause the color emitted tobecome much more blue or yellow, depending on the encapsulant material.Though this effect can be undesirable since the nanocrystals emit whitelight on their own, this additional emission allows for tuning of thelight to become cooler (more blue) or warmer (more yellow) in futureapplications. However, UV light absorbed by the encapsulant can affectthe longevity of the encapsulant.

5. Surface Modification of Nanocrystals

a. Phosphonic Acid Preparation

A Michaelis-Arbuzov reaction followed by acidic hydrolysis was used tosynthesize the various phosphonic acids employed and is describedbriefly as follows (FIG. 13). [Engel, R., Synthesis of Carbon-PhosphorusBonds. CRC Press, Inc.: Boca Raton, Fla., 1988; p 229.; Kosolapoff, G.M., Organophosphorus compounds. Wiley: New York, N.Y., 1950; p 376.]Molar equivalents of the bromoalkane of interest and TEP were refluxedat ˜150° C. for at least 24 hours in order to ensure near completeS_(N)2 substitution. The reaction was considered complete when theevolution of bromoethane was no longer evident. In general, a slightyellow color in the reaction was also noted. The reaction mixture wascooled to 80° C., followed by the addition of 2.1 molar equivalents HCl.In a few cases a small amount of deionized water was added to thereaction mixture in order to increase the overall reaction volume.Post-hydrolysis, this excess water was removed via vacuum distillation.The reaction was then refluxed for at least 48 hours at 103° C.; in thecases of the longer chain phosphonic acids, solid product was notedfalling out of solution upon completion of the hydrolysis. The reactionwas then cooled, and the product phosphonic acid was recrystallizedusing acetonitrile, ethyl acetate, or hexanes. The crystals werecollected via vacuum filtration, washed with cold solvent, and allowedto dry. The collected crystals were then placed under vacuum to removeany remaining solvent.

The purity of the phosphonic acid product was verified using nuclearmagnetic resonance spectroscopy (NMR), infrared spectroscopy (IR),negative mode electrospray ionization mass spectrometry (ESI-MS), andmelting point measurements (Supplemental information). Using negativemode ESI-MS for the phosphonic acids commonly gave dimer, trimer, andsodiated peaks in addition to the phosphonic acid minus one peak. The IRspectra of each phosphonic acid showed alkyl stretches corresponding tothe carbon chain, P—O—H stretches in the 1040-910 cm⁻¹ region, P═Ostretches in the 1200-1100 cm⁻¹ region, in addition to the 2350-2080,1740-1600, and 1040-917 cm⁻¹ peak regions, common to all phosphonicacids. [Nakanishi, K.; Solomon, P. H., Infrared Absorption Spectroscopy.2nd ed.; Holden-Day, Inc.: San Francisco, Calif., 1977; p 287.;Nakamoto, K., Infrared and Raman Spectra of Inorganic and CoordinationCompounds. John Wiley and Sons: New York, N.Y., 1978; p 448.; Gunzler,H.; Gremlich, H. U., IR Spectroscopy. Wiley, VCH: Weinheim, Germany,2002; p 361.] Proton and carbon NMR spectra were consistent with theexpected number, location, and integration of peaks.

b. Nanocrystal Preparation

Nanocrystals were synthesized with slight modifications to previouslypublished syntheses. [Peng, Z. A.; Peng, X. G., Formation ofhigh-quality CdTe, CdSe, and CdS nanocrystals using CdO as precursor.Journal of the American Chemical Society 2001, 123, (1), 183-184.;Bowers, M. J.; McBride, J. R.; Rosenthal, S. J., White-light emissionfrom magic-sized cadmium selenide nanocrystals. Journal of the AmericanChemical Society 2005, 127, (44), 15378-15379.; Gosnell, J. D. et al.,Eds. SPIE: 2006; p 63370A.] Since phosphonic acids have been found to bean impurity in tri-octyl phosphine oxide (TOPO), [Peng, X. G.; Manna,L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A.P., Shape control of CdSe nanocrystals. Nature 2000, 404, (6773),59-61.] the synthesis was modified to use only HDA as the surfactant. Ina three-neck, round bottom flask 10 g HDA, 1 mmol CdO, and 2 mmolphosphonic acid were heated to 145° C. under Ar purge. The flaskcontents were then heated to 325° C. under Ar with vigorous stirring. Asthe CdO and phosphonic acid converted to a Cd-phosphonate complex, theflask's contents became clear and colorless. After cooling to 310° C., 4mL of a 0.2 M Se:TBP solution were quickly injected into the flask. Toproduce nanocrystals small enough to have band-edge absorption featuresless than 420 nm, this initial injection was rapidly followed by asecond injection of 20 mL of butanol (kill-shot) to cool the reaction to˜130° C., followed by further cooling to below 100° C. using compressedair applied to the outside of the flask. [Bowers, M. J.; McBride, J. R.;Rosenthal, S. J., White-light emission from magic-sized cadmium selenidenanocrystals. Journal of the American Chemical Society 2005, 127, (44),15378-15379.] For the larger nanocrystal sizes, the reaction was allowedto proceed until the desired size was achieved and subsequently cooledin the same way. Small aliquots (<500 μL) were taken from each reactionand diluted in toluene. These “dirty” nanocrystals were reserved forquantum yield (QY), UV-VIS absorption, and PL testing.

Unreacted precursor and solvents were removed from the nanocrystals viaa three wash process. Initially, 15 mL of each reaction mixture wasplaced into a centrifuge tube and precipitated with methanol. Thesupernatant was discarded and the pellet was re-suspended in 8 mL ofhexanol. This suspension was centrifuged in order to remove HDA andCd-phosphonate. The supernatant (containing the nanocrystals) was thendecanted into clean tubes, and the initial methanol/centrifugation stepwas repeated. The supernatant was again discarded and the nanocrystalssuspended in toluene for further measurements, these nanocrystalshereafter will be referred to as “clean” nanocrystals.

c. Quantum Yield Determination

In order to determine the quantum yield of the nanocrystals synthesized,the emission of nanocrystals in toluene, dirty and clean, was comparedto a standard dye. At nanocrystal sizes with band-edge absorption of400-420 nm (ultra-small nanocrystals), the nanocrystal solutions werediluted to have an absorption <0.1 at 375 nm. A solution of Coumarin152A in hexanes (QY=1.00) [Bowers, M. J.; McBride, J. R.; Rosenthal, S.J., White-light emission from magic-sized cadmium selenide nanocrystals.Journal of the American Chemical Society 2005, 127, (44), 15378-15379.][Nad, S.; Kumbhakar, M.; Pal, H., Photophysical properties ofcoumarin-152 and coumarin-481 dyes: Unusual behavior in nonpolar and inhigher polarity solvents. Journal of Physical Chemistry A 2003, 107,(24), 4808-4816.] was diluted to the same optical density. Emissionspectra were then collected of each solution using 375 nm as theexcitation wavelength. For the ultra-small nanocrystals, the emissionintensity was integrated and compared to that of the Coumarin 152Asolution. Larger nanocrystal sizes with band-edge absorption featuresfrom 540-570 nm were diluted to have an optical density <0.1 at 395 nm,the excitation wavelength used for the PL. These “traditional”nanocrystals were compared to a solution of Coumarin 503 in ethanol(QY=0.56). [Reynolds, G. A.; Drexhage, K. H., New Coumarin Dyes withRigidized Structure for Flashlamp-Pumped Dye Lasers. OpticsCommunications 1975, 13, (3), 222-225.] The quantum yield of thenanocrystal solutions was then found using Equation 1:

$\begin{matrix}{{QY}_{NC} = {\left( \frac{E_{NC}/A_{NC}}{E_{STD}/A_{STD}} \right) \times \left( \frac{\left( \eta_{NC} \right)^{2}}{\left( \eta_{STD} \right)^{2}} \right) \times {QY}_{STD}}} & (1)\end{matrix}$

In this equation E_(NC) is the nanocrystals integrated emissionintensity, A_(NC) is the nanocrystals optical density at the excitationwavelength, E_(STD) is the standard's integrated emission intensity,A_(STD) is the standard's optical density at the excitation wavelength,η_(NC) is toluene's refractive index, η_(STD) the standard's solvent'srefractive index, and QY_(STD) is the standard's literature quantumyield. All QY measurements were performed on six different batches ofnanocrystals for each phosphonic acid to ensure accuracy.

In order to monitor emission pinning and avoid slight batch-to-batchvariations, an average of 21 batches per phosphonic acid weresynthesized that had band-edge absorption features below 420 nm.Absorption and PL spectra were acquired for each batch and the data fitwith a Gaussian curve to determine the center wavelength for theband-edge absorption and the highest energy emission feature.

d. Results and Discussion

Initially seven phosphonic acids with unbranched alkyl chains (FIG. 13)were used to synthesize broad band-emitting nanocrystals. These earlyexperiments were intended to show pinning of the emission ofnanocrystals with diameters less than ˜1.7 nm, using differentphosphonic acids compared to the pinning previously seen with dodecylphosphonic acid. [Dukes, A. D.; Schreuder, M. A.; Sammons, J. A.;McBride, J. R.; Smith, N. J.; Rosenthal, S. J., Pinned emission fromultra-small cadmium selenide nanocrystals. Journal of Chemical Physics2008, 129, (12).] Plotting the wavelength of the first emission featurevs. the wavelength of the band-edge absorption clearly showed a regionof pinned emission (FIG. 14). It is important to note the slope changefrom the traditional size regime of nanocrystals (>1.7 nm) to theultra-small nanocrystals. In the traditional nanocrystal sizes theemission red-shifts with red-shifting band-edge absorption andincreasing diameter. For the ultra-small nanocrystals, the differentsizes do not show a change in the wavelength of the emission from thehighest energy trap state, despite changes in the wavelength of theband-edge absorption feature. [Dukes, A. D.; Schreuder, M. A.; Sammons,J. A.; McBride, J. R.; Smith, N. J.; Rosenthal, S. J., Pinned emissionfrom ultra-small cadmium selenide nanocrystals. Journal of ChemicalPhysics 2008, 129, (12).] The average and standard deviation for thewavelength at which the first emission feature was located were obtainedfor each set of nanocrystal batches made with different phosphonicacids.

It was noted during the synthesis that it was possible to make thesmallest nanocrystals, as determined using Peng et al.'s nanocrystalsizing equations, [Yu, W. W.; Qu, L. H.; Guo, W. Z.; Peng, X. G.,Experimental determination of the extinction coefficient of CdTe, CdSe,and CdS nanocrystals. Chemistry of Materials 2003, 15, (14), 2854-2860.]with the shortest chain phosphonic acid. Table 3 lists the shortestwavelength absorption feature achieved with each phosphonic acid. Thesesmallest sizes for each phosphonic acid were achieved by reducing thetime between the Se:TBP injection and the “kill-shot” describedpreviously. The time between the injections was approximately the same(˜1 second) for all the smallest sizes, yet different sizes ofnanocrystals were achieved. This trend indicated that the shorter thealkyl chain on the phosphonic acid, the slower the reaction wouldproceed. The slower reaction kinetics allowed for smaller sizes to beobtained using the short chain phosphonic acids than were achievablewith the longer alkyl chains.

TABLE 3 Shortest wavelength band-edge absorption obtained for eachphosphonic acid Bluest λ band-edge Phosphonic acid name absorptionobtained (nm) Butyl PA 359 Hexyl PA 371 Octyl PA 373 Decyl PA 376Dodecyl PA 384 Hexadecyl PA 395 Docosyl PA 392 2-ethyl,hexyl PA (2-EHPA)378 3-methyl,butyl PA (3-MBPA) 371 Phenyl PA 374 3-phenyl,propyl PA(3-PPPA) 376

The growth kinetics of the CdSe nanocrystals depend on the reactivity ofthe Cd and Se precursors. [Peng, Z. A.; Peng, X. G., Nearly monodisperseand shape-controlled CdSe nanocrystals via alternative routes:Nucleation and growth. Journal of the American Chemical Society 2002,124, (13), 3343-3353.; Qu, L. H.; Yu, W. W.; Peng, X. P., In situobservation of the nucleation and growth of CdSe nanocrystals. NanoLetters 2004, 4, (3), 465-469.] Intuitively, the rate at which Cd atomsare added to the initial nanocrystal seeds is dependent on the rate theCd-phosphonate can be separated, generating a reactive Cd species; thisrate should depend on the strength of the Cd—O—P bond. Accordingly, itwas hypothesized that the shorter ligands would bind the strongest to Cdin the Cd-phosphonate precursor and to the nanocrystal surface.

In addition to influencing the nanocrystal growth kinetics, one wouldexpect the Cd—O—P bond would affect the energy of a Cd surface trapstate. In a previous study, Whaley et al. calculated that the density ofstates for the Cd rich surface of a nanocrystal would be affected by thebonding of the Cd atoms on that surface, including any dangling orbitals(Cd surface trap sites). [Hill, N. A.; Whaley, K. B., ATheoretical-Study of the Influence of the Surface on theElectronic-Structure of Cdse Nanoclusters. Journal of Chemical Physics1994, 100, (4), 2831-2837.] The trend that was observed in the currentexperiments was that the shorter alkyl phosphonic acids blue-shift oneof the emission features. This trend was seen after locating theshortest wavelength (highest energy) emission feature of the white-lightemitting nanocrystals for each of the phosphonic acids (FIG. 15). Eachof these ligands gives a statistically (99% confidence level) differentemission wavelength, except dodecyl, hexadecyl, and docosyl phosphonicacid. These three phosphonic acid ligands give the nanocrystals' astatistically similar emission feature, and by association the sameemission energy level. This is not surprising since an energeticdifference seen between chain lengths of 12, 16, or 22 carbons wouldindicate the trap-states interact with atoms through more than 14 bonds.Although, any trend past 3 bonds is surprising in and of itself.

Four branched chain phosphonic acids were also evaluated. The pinningresults from these phosphonic acids indicated that it is not only thenumber of carbons in the alkyl chain that is important; the arrangementof these carbons also has an influence on the wavelength of the lightemitted from the nanocrystals. Table 4 illustrates the pinnedwavelengths for all the phosphonic acids tested. Of particular note,although 3-methyl, butyl PA, octyl PA, and 3-phenyl, propyl PA all havedifferent numbers of carbons in their alkyl chain, the wavelength of thepinned emission they cause are indistinguishable.

TABLE 4 Pinned emission λ, of nanocrystals made with each phosphonicacid Phosphonic acid Emission λ (nm) Butyl PA 425.8 ± 2.1 Hexyl PA 430.2± 2.8 Octyl PA 436.3 ± 3.5 Decyl PA 440.0 ± 2.3 Dodecyl PA 445.1 ± 2.4Hexadecyl PA 445.1 ± 3.8 Docosyl PA 445.2 ± 2.9 2-EHPA 441.3 ± 4.03-MBPA 434.8 ± 2.9 Phenyl PA 429.1 ± 1.7 3-PPPA 435.1 ± 2.6

Without wishing to be bound by theory, the effect seen in the opticalproperties of ultra-small nanocrystals is believed to be a consequenceof electronegativity. The electronegativity of individual atoms affectsthe strength of bonds between atoms. In addition, the electronegativityof individual atoms is influenced by the atoms that surround the atom ofinterest. This theory of electronegativity modification is known asinductive electronegativity and is a large factor in many analyticalmeasurements, such shielding and de-shielding in NMR spectroscopy.[Jacobsen, N. E., NMR Spectroscopy explained: simplified theory,applications, and examples for organic chemistry and structural biology.Wiley-Interscience: Hoboken, N.J., 2007; p 668.; Kemp, W., OrganicSpectroscopy. 2nd ed.; Macmillan: 1987; p 299.] Using Bratsch's “GroupElectronegativity Method,” [Bratsch, S. G., A Group ElectronegativityMethod with Pauling Units. Journal of Chemical Education 1985, 62, (2),101-103.] the electronegativity of each phosphonic acid with an attachedCd was determined. First, the ligands were broken into groups of ethylcarbons, methyl carbons, branched carbons, hydroxyl groups, phosphorousatoms, and single oxygens (FIG. 16). The electronegativity (X_(G)) wascalculated for each of these groups using Equation 2, where N_(G) is thenumber of atoms in the group, υ is the number of a specific type ofatom, and χ is the electronegativity of each atom in Pauling units. Theindividual group electronegativities were then combined using Equation 3to determine the ligand's overall equalized electronegativity (X_(eq)),where N is the total number of atoms and N_(G) is the number of atoms ineach group.

$\begin{matrix}{X_{G} = \frac{N_{G}}{\sum\frac{v}{\chi}}} & (2) \\{X_{eq} = \frac{N}{\sum\frac{N_{G}}{\chi_{G}}}} & (3)\end{matrix}$

Inductive electronegativity generally is not thought to have an effecton atoms more than a few bonds away. However, for electron traps onsurface Cd atoms affected by the phosphonic acid ligand, this does seemto be the case. When the experimental data set and theoretical data setwere normalized to the each set's respective butyl PA ligand energy andsetting this value to one, this theoretical data matched up well withthe experimental emission data for the straight chain ligands and twobranched chain ligands (FIG. 17). For the two ligands tested thatincluded phenyl groups, this theoretical treatment did not yieldconsistent results, in all likelihood due to the conjugation in theligand's alkyl chain.

The pinned emission peak in these broad band-emitting nanocrystals isbelieved to be due to trap state emission. [Bowers, M. J.; McBride, J.R.; Rosenthal, S. J., White-light emission from magic-sized cadmiumselenide nanocrystals. Journal of the American Chemical Society 2005,127, (44), 15378-15379.; Dukes, A. D.; Schreuder, M. A.; Sammons, J. A.;McBride, J. R.; Smith, N. J.; Rosenthal, S. J., Pinned emission fromultra-small cadmium selenide nanocrystals. Journal of Chemical Physics2008, 129, (12).] Previously, surface cadmiums have been suggested aselectron traps in nanocrystals. [Lifshitz, E.; Dag, I.; Litvitn, I. D.;Hodes, G., Optically detected magnetic resonance study of electron/holetraps on CdSe quantum dot surfaces. Journal of Physical Chemistry B1998, 102, (46), 9245-9250.; Chestnoy, N.; Harris, T. D.; Hull, R.;Brus, L. E., Luminescence and Photophysics of Cds SemiconductorClusters—the Nature of the Emitting Electronic State. Journal ofPhysical Chemistry 1986, 90, (15), 3393-3399.] Additionally, it has beenshown that the trapping of charge carriers can cause emission fromdiscrete energy states. [Lifshitz, E.; Dag, I.; Litvitn, I. D.; Hodes,G., Optically detected magnetic resonance study of electron/hole trapson CdSe quantum dot surfaces. Journal of Physical Chemistry B 1998, 102,(46), 9245-9250.; Chestnoy, N.; Harris, T. D.; Hull, R.; Brus, L. E.,Luminescence and Photophysics of Cds Semiconductor Clusters—the Natureof the Emitting Electronic State. Journal of Physical Chemistry 1986,90, (15), 3393-3399.; Lifshitz, E.; Dag, I.; Litvin, I.; Hodes, G.;Gorer, S.; Reisfeld, R.; Zelner, M.; Minti, H., Properties of CdSenanoparticle films prepared by chemical deposition and sol-gel methods.Chemical Physics Letters 1998, 288, (2-4), 188-196.; Underwood, D. F.;Kippeny, T.; Rosenthal, S. J., Ultrafast carrier dynamics in CdSenanocrystals determined by femtosecond fluorescence upconversionspectroscopy. Journal of Physical Chemistry B 2001, 105, (2), 436-443.]The trapping energy of an electron on a surface Cd will be affected bythe available orbitals and the electron density around the nucleus.[Hill, N. A.; Whaley, K. B., A Theoretical-Study of the Influence of theSurface on the Electronic-Structure of Cdse Nanoclusters. Journal ofChemical Physics 1994, 100, (4), 2831-2837.; Hill, N. A.; Whaley, B.,Electronic-Structure of Semiconductor Nanoclusters—a Time-DependentTheoretical Approach. Journal of Chemical Physics 1993, 99, (5),3707-3715.] The electron density around a surface Cd will vary dependingon the electron-donating or electron-withdrawing nature of any moleculesbound to it. Accordingly, the difference in electronegativity of thephosphonic acids would explain the change in emission wavelength seen inthese ultra-small nanocrystals (FIG. 17). Experimentally, it appearsthat shorter alkane chain lengths cause higher energy emissions, due tothe increased electron withdrawing nature of these ligands exerted onthe Cd's adjacent dangling bond, previously suggested as being anelectron trap. [Hill, N. A.; Whaley, K. B., A Theoretical-Study of theInfluence of the Surface on the Electronic-Structure of CdseNanoclusters. Journal of Chemical Physics 1994, 100, (4), 2831-2837.;Lifshitz, E.; Dag, I.; Litvitn, I. D.; Hodes, G., Optically detectedmagnetic resonance study of electron/hole traps on CdSe quantum dotsurfaces. Journal of Physical Chemistry B 1998, 102, (46), 9245-9250.;Klimov, V. I.; McBranch, D. W.; Leatherdale, C. A.; Bawendi, M. G.,Electron and hole relaxation pathways in semiconductor quantum dots.Physical Review B 1999, 60, (19), 13740-13749.; Gomez, D. E.; vanEmbden, J.; Jasieniak, J.; Smith, T. A.; Mulvaney, P., Blinking andsurface chemistry of single CdSe nanocrystals. Small 2006, 2, 204-208.]

In addition to altering the emission wavelength of the ultra-smallnanocrystals, the phosphonic acid ligand on the surface affected the QY(FIG. 18, Table 5). Initially, the QY changes with phosphonic acid werebelieved to be due only to the cleaning process, used to separate thenanocrystals from the coordinating solvent and precursors. However, itwas noted that a similar trend was seen in “dirty” and “clean”nanocrystals when using the straight alkyl chain ligands. The branchedand phenyl ring containing ligands did not fit into this trend oncecleaned (Table 5). This could be explained by the conjugation in thechain, the cleaning process removing these ligands, or the steric sizeof these phosphonic acids reducing the surface coverage as compared withthe straight chain ligands. It should be noted that the ligand's stericsize is a possible cause for the QY effects, but is not likely to be thecause of the energetic differences, as discussed below.

TABLE 5 QY of nanocrystal's synthesized with each phosphonic acid QY ofsmall, QY of small, QY of large, QY of large, dirty clean dirty cleanPhosphonic acid nanocrystals (%) nanocrystals (%) nanocrystals (%)nanocrystals (%) Butyl PA 2.54 ± 0.17 1.03 ± 0.21 1.37 ± 0.18 0.37 ±0.15 Hexyl PA 2.80 ± 0.37 2.44 ± 0.31 2.68 ± 0.19 1.40 ± 0.32 Octyl PA2.59 ± 0.47 4.84 ± 0.56 6.35 ± 0.94 2.70 ± 0.50 Decyl PA 2.96 ± 0.456.10 ± 0.40 9.20 ± 0.45 4.88 ± 0.14 Dodecyl PA 5.24 ± 1.24 7.37 ± 0.769.08 ± 0.88 4.93 ± 0.29 Hexadecyl PA 7.37 ± 1.19 9.12 ± 0.50 9.26 ± 0.355.79 ± 0.66 Docosyl PA 6.91 ± 0.71 8.64 ± 0.51 8.25 ± 0.18 7.18 ± 0.182-EHPA 3.18 ± 0.83 3.98 ± 0.99 10.17 ± 1.09  9.14 ± 1.74 3-MBPA 5.00 ±1.18 0.75 ± 0.59 3.91 ± 0.69 2.55 ± 0.39 Phenyl PA 2.95 ± 0.34 0.50 ±0.29 2.21 ± 0.44 1.32 ± 0.35 3-PPPA 2.45 ± 0.77 0.22 ± 0.05 5.47 ± 0.940.38 ± 0.17

Nanocrystals in the traditional size regime (>1.7 nm) made with eachphosphonic acid demonstrated the known trend of longer emissionwavelength with increased diameter. However, for these nanocrystals, asimilar trend in the QY was seen with the straight chain phosphonicacids as in the small nanocrystals (FIG. 18, Table 5). Once again, thebranched and phenyl containing ligands did not fit with the rest of thedata.

The quantum yield is most likely impacted by the physical nature of theligands rather than the electronic properties. For the traditional sizeregime, the nanocrystals have a lower QY when cleaned because some ofthe ligands are removed, decreasing the surface passivation andincreasing the dangling bonds that trap charges. Since the emission inthis size regime is mainly from band-edge recombination, this leads to adecrease in the QY. The increased QY caused by longer chain phosphonicacids could be due to these longer chains passivating of any surfacestates with portions of the alkyl ligand chain. [Berrettini, M. G.;Braun, G.; Hu, J. G.; Strouse, G. F., NMR analysis of surfaces andinterfaces in 2-nm CdSe. Journal of the American Chemical Society 2004,126, (22), 7063-7070.; Garrett, M. D. et al., Band edge recombination inCdSe, CdS and CdSxSel-x alloy nanocrystals observed by ultrafastfluorescence upconversion: The effect of surface trap states. Journal ofPhysical Chemistry C 2008, 112, (33), 12736-12746.] For the ultra-smallsizes, the cleaned nanocrystals exhibit a higher quantum yield due tothe removal of ligands during the cleaning process, creating moreemission trap states. The low QY caused by the shorter chain phosphonicacids may be due to the stronger bond that shorter chains have with Cd,consequently they are less likely to be removed, preventing trap statesfrom being created.

Solvation dynamics and ligand steric were discarded as sources of theemission changes. If the emission changes were due to the solvationdynamics of the various ligands, then changing the nanocrystals tovarious solvents would change the wavelength at which the ultra-smallnanocrystals were pinned. When the nanocrystal emission was analyzed inhexanes, toluene, mesitylene, and chloroform the emission peakwavelengths were unchanged. Considering the data presented in Table 4and FIG. 15, allowed for the elimination of steric considerations.Assuming that the steric size of the ligand is the most important factorin determining the location of the first peak, the most bulky ligand isphenyl phosphonic acid and butyl phosphonic acid is the smallest ligand.This should put the phenyl phosphonic acid nanocrystals on one edge ofthe pinning data, with lessening steric size down to butyl phosphonicacid nanocrystals at the other extreme. However, the phenyl nanocrystalshad a pinned wavelength at ˜430 nm, almost the exact middle of the data.

These ultra-small nanocrystals likely consist of less than 25 Cd—Sepairs. At this small size, it is possible that the phosphonic acids notonly affect the kinetics of the growth reaction, but also influences thecrystal structure. Without wishing to be bound by theory, it is believedthat a change in the crystal structure is a potential cause for themodulated emission shown here; although is less likely due to the ligandexchange data previously presented. [Dukes, A. D.; Schreuder, M. A.;Sammons, J. A.; McBride, J. R.; Smith, N. J.; Rosenthal, S. J., Pinnedemission from ultra-small cadmium selenide nanocrystals. Journal ofChemical Physics 2008, 129, (12).]

e. Conclusions

The emission of ultra-small CdSe nanocrystals was shown to be pinned atdifferent wavelengths depending on the phosphonic acid used duringsynthesis. For straight chain phosphonic acids, longer alkyl chainspinned the bluest emission feature at longer wavelengths than shorterchains. Surprisingly, the energy changes are shown to correlate with theelectronegativity of the phosphonic acid ligands. In addition toshifting the wavelength of the pinned emission, these ligands are shownto influence the quantum yield of nanocrystals in both the ultra-smalland traditional size regime. The quantum yield for such white-lightemitting nanocrystals can be increased to about 10%, or even higher.

This new size regime of ultra-small nanocrystals has stimulated the ideathat the nanocrystal and ligands may no longer be considered separately.At the sizes studied here, the ligand has become essential to theoverall crystal's optical properties. This recognition requires a shiftin thinking and experimentation. At this ultra-small size, each atombecomes increasingly important regardless of whether it is part of theinner crystal, the nanocrystal surface, or the ligands. Furtherknowledge, along with controlling the placement of atoms at specificlocations in these small crystals will allow for precise alteration ofmany of the physical, electronic, and optical properties of thesenanocrystals. An advantage of utilizing the electronegativity effect isthat it allows for the fine-tuning of the white-light emission fromthese nanocrystals.

6. Example Preparation of Nanocrystals with Phosphonic Acid Ligands

a. Synthesis

In a three-neck, round bottom flask 10 g hexadecylamine, 1 mmol CdO, and2 mmol of hexadecyl phosphonic acid were heated to 145° C. under Arpurge. The flask contents were then heated to 325° C. under Ar withvigorous stirring. As the CdO and phosphonic acid converted to aCd-phosphonate complex, the flask's contents became clear and colorless.After cooling to 310° C., 4 mL of a 0.2 M Se:tri-butyl phosphinesolution were quickly injected into the flask. To produce nanocrystalssmall enough to have band-edge absorption features less than 420 nm,this initial injection was rapidly followed by a second injection of 20mL of butanol (“kill-shot”) to cool the reaction to ˜130° C., followedby further cooling to below 100° C. using compressed air applied to theoutside of the flask.

Unreacted precursor and solvents were removed from the nanocrystals viaa three wash process. Initially, 15 mL of each reaction mixture wasplaced into a centrifuge tube and precipitated with methanol. Thesupernatant was discarded and the pellet was resuspended in 8 mL ofhexanol. This suspension was centrifuged in order to remove HDA andCd-phosphonate. The supernatant (containing the nanocrystals) was thendecanted into clean tubes, and the initial methanol/centrifugation stepwas repeated. The supernatant was again discarded and the nanocrystalssuspended in toluene for further treatments.

b. Post-Cleaning Treatment

About 125 mL of a 10 μM ultra-small nanocrystal solution and a molarexcess (2.5 mmol) of Zn acetate were added to a round bottom flask.While stirring this solution was heated at 60° C. for 15 minutes. Thesolution was cooled and a majority of the toluene was removed in vacuo(rotary evaporator). The remaining solution was divided into centrifugetubes and these were filled with methanol. After centrifugation, thenanocrystals were re-suspended in toluene for analysis.

c. Results

The nanocrystals post-synthesis had an average quantum yield of 7.4%,which increased up to 9.2% after the initial cleaning process. Thepost-cleaning treatment described above has been shown to increase thequantum yield by as much as 80%, or yielding nanocrystals with a quantumyield of nearly 17%. Some other metal acetate treatments (Cd and Mnacetate) increased the quantum yield, while others (Pb and Hg acetate)decreased the quantum yield of these ultra-small nanocrystals; however,Zn acetate showed the largest increase.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the scope or spirit of the invention. Otherembodiments of the invention will be apparent to those skilled in theart from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

What is claimed is:
 1. A method of increasing quantum yield of anultra-small nanocrystal, the method comprising the steps of: (a)providing the ultra-small nanocrystal by heating a reaction mixturecomprising a source of cadmium or zinc and a molar excess of one or moreC₄ to C₂₂ alkyl- or aryl-phosphonic acids, and adding to the reactionmixture an injection mixture comprising a C₂ to C₁₆ trialkyl- ortriarylphosphine and a source of selenium, sulfur, or tellurium; and (b)treating the nanocrystal with a molar excess of acetic acid or aconjugate base thereof.
 2. The method of claim 1, wherein the aceticacid or conjugate base thereof is zinc acetate.
 3. The method of claim1, wherein the acetic acid or conjugate base thereof is acetic acid. 4.The method of claim 1, wherein the nanocrystal comprises cadmium orzinc, crystallized with selenium or sulfur.
 5. The method of claim 1,wherein the nanocrystal comprises cadmium selenide.
 6. The method ofclaim 1, wherein the nanocrystal is a broad emission quantum dot.
 7. Themethod of claim 1, wherein treating is performed for time period is atleast about 15 minutes.
 8. The method of claim 1, wherein treating isperformed at a temperature of at least about 60° C.
 9. The method ofclaim 1, wherein treating is performed at least about 60° C. for atleast about 15 minutes.
 10. A method of increasing quantum yield of anultra-small nanocrystal, the method comprising the steps of: (a)providing the ultra-small nanocrystal by heating to a temperature ofgreater than about 300° C. a reaction mixture comprising a C₄ to C₂₂alkyl- or aryl-phosphonic acid and a source of cadmium or zinc in amolar ratio of from about 1:5 to about 1:1 Cd/Zn:phosphonic acid, addingto the reaction mixture an injection mixture comprising a C₂ to C₁₆trialkyl- or triarylphosphine and a source of selenium, sulfur, ortellurium, and decreasing the temperature of the reaction mixture toless than about 300° C.; and (b) treating the nanocrystal with a molarexcess of acetic acid or a conjugate base thereof.
 11. The method ofclaim 10, wherein the acetic acid or conjugate base thereof is zincacetate.
 12. The method of claim 10, wherein the acetic acid orconjugate base thereof is acetic acid.
 13. The method of claim 10,wherein the nanocrystal comprises cadmium or zinc, crystallized withselenium or sulfur.
 14. The method of claim 10, wherein the nanocrystalcomprises cadmium selenide.
 15. The method of claim 10, wherein thenanocrystal is a broad emission quantum dot.
 16. The method of claim 10,wherein treating is performed for time period is at least about 15minutes.
 17. The method of claim 10, wherein treating is performed at atemperature of at least about 60° C.
 18. The method of claim 10, whereintreating is performed at least about 60° C. for at least about 15minutes.